1979. American diver Greg Louganis, in a portentous moment foreshadowing Seoul nine years later, strikes his head on the springboard at the Olympic trials in Moscow. He blames jet lag for upsetting his coordination.
1992. Skater Tonya Harding attributes her disappointing fourth-place finish at the Olympic Games in Albertville to jet lag, another complaint on her list of saboteurs that have upset her performances, including traffic jams, the back of her dress coming unhooked, a loose skate blade, a broken shoelace, and an assassination threat.
2000. U.S. world and Olympic champion sprinter Maurice Greene, once the world’s fastest man, explains that jet lag was to blame for his loss to Portugal’s Francis Obikwelu in the 100 meters at the Gaz de France track meet. In 2002, he withdraws from the Grand Prix 100-meter event in France, citing jet lag as responsible for his mediocre time of 0:10.56 in the heats. In 2004, he again loses to Obikwelu in France, complaining that his body was very tired from his overseas flight.
That’s just a bit of athletic folklore about jet lag. In the preceding examples, was jet lag really to blame? Or was it just a handy excuse? When traveling across multiple time zones to compete, international athletes can disturb their regular biological rhythms. This physiological offset might lead to disastrous impairment in their performance on the world athletic stage. (In this regard, they would share infamous episodes of vomiting, fainting, and questionable judgment induced by jet lag that have also embarrassed political leaders during international voyages.) But some experts don’t think so. All athletes in international competition must understand and adjust for travel-induced interruptions of their biological rhythms. But how much does jet leg truly affect performance? No one knows for sure. This chapter is devoted to the question.
By now, we should be convinced that we humans abound in timekeepers. We’ve seen in the previous chapters that our internal clocks finely tune the sequence of muscular innervation that permits locomotion. They guide our pace in races of different distances and they help us parcel out the rate of effort during different phases of competition. Without our conscious effort, these smart biological timepieces adjust, for reasons not entirely clear, the tempo of oscillations of motor activity during exercise to some optimal rate. And, when called upon to deliver maximal effort, they can rev up the muscle motor to top speed. Even the pattern of variability in stride rate or length during running and walking appears to be under the control of some reliable timepiece. It’s a good thing that the alarms in all these clocks don’t go off every hour.
It turns out, though, that even bigger timekeepers must be considered. In fact, every living being possesses clocks that govern phasic increases and decreases in cell function that cycle with chronological time. Since such patterns usually approximate a day’s duration, they are termed circadian rhythms, from the Latin circa (about) and dies (day). They’re evident throughout both the plant and animal kingdoms, indicating that these rhythms are a central feature of living matter. However, why our tissues do their work with an eye on the clock (unlike you or me, of course) remains an unsolved mystery.
The clocks that dictate circadian rhythms have no particular regard for athletic competition. But they do govern phasic changes in physiological functions that have important bearing on motor performance. Muscle strength, thermoregulatory mechanisms, cardiac output, even motivation all naturally and predictably change throughout the course of a day. So, the time of day when athletes train or compete may be critically important. We should expect that particular times of the day (or night) will optimize performance outcomes. Recognizing the characteristics of circadian rhythms as they relate to motor performance, then, just might prove beneficial to the competing athlete.
This chapter first briefly reviews what we know about circadian rhythms, their nature and mechanism. It then considers how these rhythms influence both physiological variables and specific outcomes of motor performance. Finally, it explores what happens to athletic performance when circadian rhythms are disturbed, such as the problem of jet lag when athletes travel great distances to compete.
Paris, the summer of 1814, about 8:00 in the evening. Julien-Joseph Virey has just finished his duties as head pharmacist at the Val-de-Grâce hospital, and he is now roaming the hospital wards. “How many have died today?” he asks the clerk. “What time did they pass away?” He carefully records the answers in his notebook.
Professor Virey was working on his doctoral thesis, which he eventually titled “Ephemerides of the Human Life or Research on the Daily Revolution and the Periodicity of Its Phenomena in Health and Diseases.” In this work, he presented a catalogue of known observations of 24-hour rhythms in both plants and animals and added his own observations of the daily patterns of patient deaths. He found these to demonstrate regular peaks of incidence at 6:00 a.m. and 10:00 p.m. This pattern has been replicated by contemporary studies as well.
Such regular biological oscillations had been observed by many before Virey’s time, but his thesis was so significant (and unpopular with his peers) because of his claim that these rhythms were intrinsic to the organism, rather than simply a response to external stimuli. Since he turned out to be correct, it has been suggested that he should be credited as the true founder of the field of chronobiology, the science of biological rhythms.1 This distinction came too late, however, since he died in Paris in 1846, unappreciated by his colleagues.
(Professor Virey was, in fact, not very popular with his peers, even when alive. Though obviously highly intelligent—he was also a naturalist, anthropologist, author, and philosopher—he never gained much favor with the scientific community. That was reportedly due mainly to his “naïve and wordy writing,” indecisiveness, poor reasoning skills, and unpopular ideas. Indeed, one of the reviewers of his thesis commented, “This is the way, Mr. Virey, that one delivers medicine to public mockery and to the scorn of scholars.”1)
What was Virey talking about? Suppose you recognize that body temperature varies periodically and predictably over the course of a day, increasing during the day and falling at night. You might conclude, not unreasonably, that this rhythmic oscillation is simply a response of the brain’s thermoregulatory centers to sleeping, fatigue, or maybe ambient temperature. But Virey contended that an endogenous rhythm to body temperature exists that is located within the organism, independent of outside forces. His opponents countered that these changes are simply due to the influences of external stimuli.
This debate went on for a good many years, and it wasn’t until more than 100 years later that another Frenchman, a 23-year-old geologist named Michel Siffre, finally provided proof of the intrinsic nature of biological rhythms. Siffre figured that the best way to find the answer was to see what happened to body rhythms when a human was totally isolated from environmental influences, or even from the knowledge of what time it was. So, in 1962, he descended 375 feet (115 m) beneath the ground to spend two months by himself, camped out in a tent on an underground glacier in the Scarasson cavern in northern Italy. There in the darkness and cold, he was fully unaware of time, totally isolated from humanity except for a daily telephone call to supporters at the surface. His goal, besides geological research, was to see if his perceptual notion of time, governed by his physical needs for sleeping and eating, would be altered or dissociated from actual clock time. “My aim,” he said, “was mainly to accomplish a kind of subterranean ‘hibernation’ that would fulfill a new research in human psycho-bio-physiology.” (A lofty goal to be sure. Detractors claimed it was nothing but a self-engrandizement stunt.)2
For 60 days, he suffered bitter cold, dripping humidity, dark, loneliness, and terrifying rock slides, without any knowledge of time, day, or night. His diary records increasing bouts of depression, memory loss, visual hallucinations, dizzy spells, and severe back pain. (One entry about seeing pictures of his old girlfriends shows he was getting a bit nutty: “Farewell, dear girls. Farewell, the memories of my sentimental life. How I would like to hear your voices. Time no longer means anything to me.”)
After he was dragged semiconscious to the surface when his ordeal ended, his estimation of duration for events in the cavern had become wildly distorted. But what had not changed, and in fact had remained quite steady, was his nycthemeral rhythm, the periodic duration of combined waking and sleeping periods within 24 hours. That is, even though one sleeping period might be much shorter than another, he compensated by making changes in the time he was awake. The overall combination of time spent awake and asleep did not change. Not only that, but the duration of this nycthemeral rhythm was a bit longer than a day at 24 hours and 31 minutes. With no clues whatsoever of clock time as measured by the rotation of the earth or light-dark cycles of day and night, his body had kept to a strict timetable. However, his was slightly longer than the solar day. Here was evidence in humans, previously indicated in plants and animals, of the intrinsic nature of circadian rhythms.
Later on, too, it was hard to argue against those who were clever enough to demonstrate the actual site of the pacemaker in animals that drove biological rhythms (the suprachiasmatic nucleus of the brain, for example) and to later identify particular genes that directed the running of the clock. Such rhythms were clearly generated by innate biological timekeepers.
With this point settled, the field of chronobiology took off. Aspects of biological rhythms have since become important to fields as diverse as medical practice, molecular genetics, ecology, engineering, and neuroscience. Indeed, another scientific field that encompasses as many different disciplines under its umbrella probably doesn’t exist. No fewer than 14 international societies are devoted to the study, and workers in this field can consult several journals that exclusively publish articles about biological rhythms. According to a Medline search, 20,603 articles were published with the key words circadian rhythms between 1996 and 2008. The field is immense, filled with both intriguing scientific mystery and promise. The following section touches just a small portion of it.
Rhythmicity—a regular, predictable, phasic variation over time—has been observed in the functions of virtually every living being in which it has been investigated. Single-celled organisms, like the gonyaulax, demonstrate rhythmic bioluminescence as they glow on the ocean’s surface. Crickets chirp in diurnal rhythms to attract a mate. Bears are programmed to hibernate in cyclical periods with the seasons.
Most biological rhythms in humans are diurnal (that is, their cycles vary over a day’s time). If you are like most people, a toothache is least painful right after lunch, your zeal to reproduce peaks at 10:00 p.m., your bladder fills more slowly when you’re asleep (thank goodness), and you solve problems best at noon. Because of phasic changes in the water content of your intervertebral disks, you are tallest at 7:30 in the morning. And when the moon is closest to a position overhead, you weigh less, due to shifting gravitational effects (figure 4.1).3
All biological rhythms demonstrate two defining characteristics:
1. In a constant environment, as was observed during Siffre’s adventure, biological functions vary with an intrinsic periodicity. Most approximate 24 hours (circadian), but others can vary in respect to the tides (circatidal), the month (circalunar), the year (circannual). They may even be longer, such as the 13-year reproductive cycle of the cicada. This endogenous, phasic change in function is termed the free-running period. As we discuss later on, these intrinsic rhythms are considered to be genetically controlled. They tend to be quite precise, some varying less than a few tenths of a percent in duration.
2. The intrinsic periodicity can be changed, or entrained, to fit that relating to extrinsic, or environmental, cues. Most commonly, this is the light-dark cycle in the course of a day (photic entrainment). In the natural world, the circadian rhythm of most functions thereby shifts to a more precise 24-hour clock (figure 4.2). Some interesting nonphotic entraining agents also exist, including cyclical changes in temperature (particularly in plants), song (birds), availability of food, and even social triggers in some animals.
So, in a dark cave, isolated from the normal light cycle of night and day, body temperature waxes and wanes following the directives of an internal clock that is set to about 24 hours and 15 minutes. If you leave the cave, however, your clock gets reset to a 24-hour periodicity that is linked to light changes in a chronological, astronomical day.
It is important to note, then, that although they have an intrinsic clock, biological rhythms operate in the real world by adapting to times defined by extrinsic stimuli. The central importance of biological timekeepers lies in their ability to adapt functions of the organism to geophysical cycles in their environment, not simply to generate their own intrinsic rhythm. It would seem, then, that both sides of historic debate regarding the nature of biological rhythms were correct: Regular phasic change in biological function is an intrinsic property of all living matter, but these rhythms are altered by extrinsic sensory input. The reason for this dualistic control is obscure. In this regard, it is intriguing (indeed, very puzzling) to note that in the temporal organization of biological rhythms, regardless of cycle duration or organism, the timing of the endogenous pacemaker is almost (but not quite) the same as that to which it entrains in response to environmental cues.4
Where are these biological clocks? And who sets them? As noted before, it was only when these questions began to be answered by some clever investigators that convincing evidence emerged for the inherent nature of biological rhythms.
If you destroy the suprachiasmatic nucleus (SCN) in the brain of a rat, its circadian rhythms of drinking, physical activity, and certain hormonal functions are eliminated. And if you transplant the SCN of rat A into rat B (another fun home project), the latter develops the biological rhythms of the former. If you take out an animal’s SCN (it’s about the size of a BB) and put it in a test tube, it still continues to fire clocklike impulses. Additional studies have confirmed the SCN as the master biological pacemaker in mammals (and presumably in humans); other tissues may have subsidiary timekeepers, however. Some investigations have suggested that a great many individual cells (perhaps all of them) in mammals possess timekeeping abilities, but these are subservient to the SCN.4
These biological clocks must act to control gene expression and activity of transcribed proteins. In fact, a number of these clock-controlled genes have been identified. It is notable—and probably meaningful, in some mysterious way—that the subjects selected for such studies are common household nuisances (fruit flies, molds, and mice).
The roots of rhythmic biological behavior that are linked to geophysical events (particularly the passage of the sun in a day’s time) lie in humanity’s distant past. They remain as part of our genetic heritage. From a Darwinian perspective, then, such rhythms must be interpreted as proffering certain survival advantages during the course of evolutionary progression. But what are they?
In many examples of biological rhythms, this is obvious. Reduction of metabolic demands coincide with a period of decreased food availability (hibernation). Intrinsic clocks adjust feeding times when food is accessible (honeybees). The same timekeepers are used for celestial navigation during bird migration. Serum glucocorticoid levels increase during times of high physical activity (rodents). The function of internal rhythms in response to geophysical events allows animals to anticipate, rather than simply respond to, these critical periods.
Other rhythms are harder to explain. Why should joint flexibility be greatest between 4:00 and 6:00 in the afternoon? Why does the stickiness of blood platelets peak just when you’re getting up in the morning? Why can most people concentrate better between 10:00 a.m. and noon? Perhaps some clocks represent only vestigial remnants of functions that once upon a time bore importance.
The meaning of free-running biological rhythms remains a mystery, too. After all, we do not live in caves. And why do intrinsic rhythms (almost) match the entrained rhythm of chronological time (where the advantages of biological rhythms seem to be most evident)? Much has been learned about biological rhythms, but many mysteries remain.
Although biological rhythms are presumably expressions of adaptive mechanisms from our dim evolutionary past, their effect on behavior and physiology clearly has bearing for present human welfare. These diurnal variations help us consider the safety and performance of military personnel, shift workers, long-distance truck drivers, resident physicians, and astronauts in space. It has been contended that circadian rhythms (or particularly their interruption) have contributed to human disasters (which characteristically occur at night), such as the nuclear accidents at Chernobyl and Three Mile Island, the Exxon Valdez oil spill, and traffic fatalities. Human illness also exhibits circadian rhythmicity. Strokes and heart attacks occur most frequently between 6:00 a.m. and noon, asthma symptoms are worse in the evening, and certain drug treatments are more efficacious at particular times of day.
Count up the number of physiological functions that go into any particular athletic performance. Like a 10K road race, for instance. All the biochemical events that make up muscular contraction, the factors contributing to the coordination of muscular activity, the neurological innervation, the thermoregulatory mechanisms, the circulatory response, lung function, blood volume, cognitive and central command, motivation. By now, you’re beginning to lose count! Next, consider that each of these many factors is attached to an intrinsic clock that causes function to vary periodically in a regular, sine-wave fashion over a 24-hour period. They increase and decrease in a constant periodicity, but each function does not necessarily reach a peak at the same time during a solar day. The conclusion? Circadian rhythms can be expected to weigh heavily on athletic performance and maybe even respond to training. Is this, in fact, true? And, if so, how can athletes time their training regimens and schedule competitions to best take advantage of peak periods of function that determine physical capacity? The following section considers the answers to these questions.
Athletes, who are always looking for a competitive edge, have had no small interest in the potential relevance of intrinsic circadian rhythms for sports performance. The result has been a considerable volume of research over the years that has examined the biological periodicity of both physiological variables and performance outcomes. In 1997, Thomas Reilly and his colleagues at John Moores University at Liverpool published a marvelous book titled Biological Rhythms and Exercise (Oxford University Press) that reviewed the published information on this subject. Updates have been forthcoming from this group since.5 These publications, which are highly recommended, are available to readers who wish a detailed assessment of the research literature. I will endeavor to summarize what we know about biological rhythms and sports performance, supplementing those reviews with data from studies that are even more recent.
As we survey this information, it will be apparent that diurnal patterns of physiological and performance markers do appear to exist. But some important questions need to be considered:
• Do temporal peaks in function and performance truly reflect endogenous biological clocks, or are they simply responses to extrinsic environmental factors?
• If they are intrinsic in origin, are these biological changes of sufficient magnitude to bear importance to athletes compared to the influences of controlled factors, such as training regimens, equipment, and diet?
• How much individual variability exists in performance-related biological rhythms? Can findings in group data necessarily be applied to specific athletes?
It will become immediately evident upon perusing these data that the answers to such questions are not entirely at hand. Obviously, though, if recognizing phasic changes in physiological function and sports performance is useful for athletes, these insights are needed.
Virtually every form of physiological function demonstrates temporal rhythmicity. Most forms are circadian in nature, rising and falling in a sine wave throughout the course of the solar day. Such rhythms can thus be characterized not only by their duration but also by the amplitude of these phasic swings (how much the factor changes over the course of a day), as well as by the times of greatest and lowest activity during a 24-hour period.
Most of the physiological factors that contribute to physical performance demonstrate a peak activity in the late afternoon and early evening. It has been suspected that such patterns are driven by their link to phasic changes in core body temperature. Psychological variables that are expected to influence sports performance, on the other hand, tend to peak earlier in the day. In considering the timing of peak activity of such variables, then, one is confronted with the extraordinary complexity of the critical determinants of athletic performance.
Core body temperature is normally regulated within narrow limits by hypothalamic centers in the brain that control gain and loss of body heat. During the course of 24 hours of normal life activities, however, measurements by rectal thermometer or gastrointestinal pill telemetry indicate a regular diurnal change in this temperature. If you are like most people, you are warmest around 6:00 p.m., with a temperature of around 37.2 degrees C. Levels then decline until you are coolest (about 36.3 degrees C) early in the morning while you’re still asleep (4:00 a.m.).
This rhythmic change in core temperature reflects the combined effects of at least three factors: one internal variable (an endogenous biological rhythm driven by the SCN) and the combined influences of two extrinsic variables (the cycle of sleeping and waking and elevations in metabolic processes as you perform physical and mental tasks). To determine the relative influence of the intrinsic biological clock itself, you can experimentally place subjects in a constant routine, preventing them from sleeping and keeping them quiet and sedentary. When you do this, the daily rhythm of core temperature is unchanged, but the amplitude of 24-hour variation is somewhat reduced.
This information has led investigators to consider the circadian rhythm of core temperature one of the best examples of endogenous biological rhythms and, more specifically, an indirect indicator of SCN activity (the latter being particularly difficult to study in intact humans!). Some have suggested, too, that circadian rhythms of core temperature may be the driving force behind other physiological rhythms that peak at the same time of day, since body temperature has a direct influence on a number of these. Moreover, in a number of cases, such a link might be expected to improve motor performance. Velocity of nerve conduction, for example, increases by 2.4 meters per second for each rise in body temperature of 1 degree C. Enzymatic activity in metabolic processes is accelerated as temperature increases. Elevations in body temperature may positively influence flexibility and contractile function of muscles and tendons.
On the other hand, we know that a rise in total body temperature, particularly when combined with high ambient-heat conditions, not only impairs performance in sustained exercise, but also poses a risk for heat injury (heat exhaustion, heat stroke). Accumulation of body heat is an anathema to the distance athlete. Anything that would add to body heat, be it a hot July afternoon or a circadian peak in core temperature, would be disadvantageous to a 10K finish time.
Thinking ahead to the next section on performance, then, you might predict that, based on a higher muscle temperature, you’d do best in short-burst activities like sprinting during the early evening. But if periodicity in sustained forms of exercise depends on the rhythm of core temperature, the opposite should be seen in aerobic endurance sports. Daily periodicity of performance in the heat in distance events (running and cycling), which is negatively affected by rises in core temperature, might be predicted to be opposite to that of circadian rhythm of core temperature. Best performances would be in the morning.
It’s obvious that these conclusions are a gross oversimplification of conditions in the real world. Here, we see the difficulties faced by researchers in separating out the relative magnitude of effects of intrinsic and extrinsic factors in defining circadian rhythms of motor performance. In this case, for example, the ambient temperature will probably be considerably lower in the morning, which might far outweigh the influence of circadian variations in core temperature. And, just to further complicate matters, a circadian rhythm also exists in the sensitivity of the hypothalamus to changes in core temperature. As core temperature rises, the magnitude of its response in turning on cooling mechanisms (sweating rate, cutaneous vasodilatation) varies over the course of 24 hours. Such responses are greater during waking hours than during sleep. They are also more pronounced during peak exercise than at trough times of circadian fluctuations in core temperature. There exists evidence, too, that changes in environmental temperature can alter both the periodicity and amplitude of circadian rhythms. Confused yet?
Anyway, it will be interesting to see how this all pans out in the section of this chapter that examines the results of studies evaluating circadian rhythms in actual sports performance. Is the daily swing in core temperature a central factor for defining similar circadian variations in sports performance? Or do temporal changes in performance listen to other physiological clocks? How critical are circadian changes in body temperature to strategies for optimizing athletic success?
The data regarding circadian rhythms in force of muscle contraction are generally much more straightforward, being less affected by extrinsic factors. Reilly and colleagues reviewed seven studies that examined isometric strength (grip, back strength, leg extension) or dynamic-isokinetic knee strength in relation to different times of the day.5 Consistently, these reports indicated that peak strength was exhibited between 5:00 and 7:00 p.m. (corresponding to peaks in core temperature) and that the amplitude was more than trivial (ranging from 4 to 10% of the average value).
A report by Nicolas and colleagues provided a somewhat different view of this issue. In this study, 12 adult males performed a series of 50 maximal contractions of the knee extensor muscles at a constant angular velocity (2.09 radians per second) at 6:00 a.m and again at 6:00 p.m. Expected maximal torque values were 7.7% greater at the later time. However, the rate of decline of strength with repeated contractions (that is, greater fatigue) was observed in the evening. So, by the time the subjects had achieved 20 repetitions, torque values were similar for the two testing times.6
The athlete’s ability to utilize oxygen is critical to aerobic endurance performance. As measured during laboratory exercise testing, the maximal aerobic power (
O2max) is a composite expression of a myriad of physiological factors that define oxygen transport and uptake. Each has its own intrinsic biological rhythm.
One’s oxygen uptake at rest (reflecting the basal metabolic rate) demonstrates clear-cut circadian rhythmicity, with the lowest levels at 4:00 a.m. and a peak in the late afternoon and early evening. At first thought, the coincidence of this peak with that of core body temperature might be expected, since metabolic rate normally responds to elevations in temperature by the Q10 effect (rise in metabolic rate by 10% for every 1 degree C rise in body temperature). However, Reilly pointed out that circadian variations in body temperature account for only 37% of those in resting O2.
During exercise, things get a bit more complicated. For example, Reilly found that when studying a single subject, O2 expressed as ml/kg/min while cycling at 150 watts peaked at 2:40 in the afternoon. However, with closer analysis, it was apparent that this variability was entirely due to circadian changes in body mass (the size-normalizing denominator) instead of O2. But then, absolute O2 was found to exhibit periodicity at lower work rates.
Identification of diurnal rhythms of O2 during submaximal exercise may depend on the exercise model being employed. Some have found no periodicity to O2 during submaximal work while others have, with a peak between 2:00 and 5:00 in the afternoon, found an amplitude of 13%.
No circadian variability has been observed in O2max. However, as Reilly and colleagues pointed out in their book, this is a particularly difficult question to sort out. Variations in test-retest O2max are influenced by subject motivation as well as variability of equipment measurement. These variations might well mask true endogenous biological rhythms in maximal aerobic power. If such intrinsic rhythms exist in maximal aerobic power, they are presumably of small amplitude. Reilly and colleagues noted that the amplitude of the circadian rhythmicity of O2 at maximal exercise, if reflecting that at rest, would be about 0.5%. (Who knows, however, the extent that even such small, circadian rhythmic changes in physiological aerobic fitness might have on aerobic endurance performance where seconds count?)
Circadian variability has been reported in flexibility of certain joints (lumbar flexion and extension, lateral glenohumeral rotation, whole-body forward flexion) with amplitudes approaching 20% of the average daily level. Demonstrated times of peak flexibility in these rhythms vary considerably between subjects, but typically occur in the afternoon and evening hours.
Performance on a 30-second all-out Wingate cycling test provides information regarding peak and mean anaerobic power. These variables demonstrate circadian rhythmicity, with highest values between 3:00 and 9:00 p.m. and an amplitude of 8%. Similar diurnal periodicity has been observed with bench exercises as well as with stair-running and jumping tests.
Blood levels of a number of hormones that may have a bearing on sports performance demonstrate phasic circadian changes. Catecholamines (epinephrine and norepinephrine) reach their greatest values in the early afternoon. Deschenes and colleagues found rhythmicity in plasma lactate and norepinephrine responses to maximal exercise, with a nadir at 8:00 in the morning. Cyclical patterns of levels of cortisol and growth hormone, on the other hand, peak during the sleeping hours.
Investigators have utilized a number of tools to examine circadian changes in mental alertness and arousal, which obviously are critical to success in many sports. Generally, they have found that these markers of mental function demonstrate circadian variability with peak levels in the afternoon. A high level of arousal may, however, be antithetical to performance in activities requiring fine-motor control. Tasks requiring hand steadiness and the ability to balance are more successfully accomplished in the morning.
Rating of perceived exertion (RPE), which places a numerical value to subjective feelings of fatigue during exercise, is said to demonstrate circadian rhythmicity with lowest levels (best psychological tolerance for exercise) in the late afternoon and early evening. It is difficult, however, to sort out whether cyclical changes in perception of effort reflect circadian variability of the physiological responses that trigger such feelings (heart rate, ventilation, metabolic rate), or whether independent, intrinsic biological variations in the central nervous system functions make us feel fatigued.
Reaction time to auditory or visual stimuli is greatest in the early evening hours, coinciding with the time of peak core temperature. As noted previously, this may reflect the direct effect of increased velocity of nerve conduction from a rise in body temperature. It is interesting, though, that faster reactions are apparently gained at the expense of accuracy, which is worse in the early evening.
How about temporal changes in the ability to strategize? No one knows, but processes like the ability to perform arithmetic and short-term memory are optimized in the early morning hours. Long-term memory may be different. One study found that school children’s memory recall was 8% greater one week after material was presented to them at 3:00 p.m. than at 9:00 a.m. In general, circadian rhythms of cognitive processes tend to peak earlier in the day than those of physiological variables. And, interestingly, the circadian rhythms in mental tasks requiring a high degree of memory load peak 8 hours earlier. They also have a lower amplitude than those involving less memory work.
Self-rated feelings of mood and well-being tend to be accentuated midafternoon, around 2:00. Somewhat surprisingly, circadian rhythms in such measures have been found to be only minimally influenced by whether you are a morning or night person. Temperature peak is separated by about an hour in the two groups. Morning people have higher epinephrine levels at dawn, and their mood rhythms are shifted a couple of hours. But the basic circadian rhythms are similar for both groups.
So much for the components of motor activity. With all these documented circadian rhythms—some with rather substantial amplitude—we might expect that similar periodicity would be observed in actual athletic performance. Let’s look at the findings in the limited number of available studies. These investigations represent a large variety of research models (each with unique strengths and weaknesses), and most have simply compared performance at selected times of day rather than over the course of 24 hours. So, they really should be interpreted as time-of-day studies rather than as investigations examining circadian rhythms. But that’s really alright, since athletic competitions don’t typically occur in the wee hours of the morning.
When a group of British footballers attempted to set an aerobic endurance record for five-a-side competition by playing continuously for four days, Reilly and Walsh saw an opportunity. Here was a chance to examine true 24-hour circadian variability in sports play. They measured the players’ pace of activity with motion analysis and found a circadian rhythm with a peak at about 6:00 p.m. and a trough between 5:00 and 6:00 a.m. That was, at least, up until the 91st hour of play, when the participants began to have visual hallucinations and “transient schizoid behavior” from sleep deprivation, and the refs called the match off. A negative correlation was observed between activity levels and players’ (not the authors’) subjective feelings of fatigue.7
Swimmers competing the 100-meter distance have been found to perform better at 5:00 p.m. than at 7:00 a.m. One study indicated best performances (by 3.6 and 1.9%, respectively) were observed in the evening in both the 400-meter crawl and repeated 50-meter trials.
Young cyclists have provided better times in 16K races that were conducted in the afternoon, compared to those in the morning. The pedaling rate and velocity spontaneously selected by cyclists also varies over the course of a day. In the laboratory, Deschenes and colleagues could find no relationship between aerobic endurance time during a progressive cycling test to exhaustion and time of day in fit college students. Cycling sprinting power, as demonstrated by ergometers, is greater in the evening (5:00 to 7:00 p.m.) than in the morning hours (7:00 to 9:00 a.m.).
However, in that study, when repeating such a sprint five times, the rate of decay in performance was faster in the evening. And, when all was averaged out, the total work on the five trials was independent of time of day. This pattern resembles that previously noted for repeated measures of isokinetic strength.
Marathon performance is closely linked to ambient temperature. Thus, it is not surprising that the best times are generally recorded in races conducted in the early morning. In this case, an extrinsic, environmental factor clearly masks any possible intrinsic, biological one.
Nationally ranked male competitors have reported their fastest 80-meter sprint times at 7:00 in the evening. Except for a small dip in the early afternoon, performance on multiple trials gradually improved during the course of the day.
Serve velocity of tennis players is greater at 6:00 p.m. than at 9:00 a.m. However, the inverse has been observed in serving accuracy, which is optimal in the morning. Similar findings have been described with badminton players.
In one report, evening performance was superior in most swimmers, runners, and shot putters, and for a crew of runners. Reilly and colleagues noted that during track-and-field competition in the last half-century, world records in only the men’s shot put and women’s javelin have been set in the morning. These authors also highlighted the fact that middle-distance running records set by their British countrymen (Sebastian Coe, Steve Cram, Steve Ovett, and Dave Moorcroft) in the 1980s were all achieved after tea time (between 5:00 and 11:00 p.m.), acknowledging that few 1,500-meter events are actually scheduled in the morning hours.
Youngstedt and O’Connor argued that the changes in performance during the course of the day (outlined in the preceding section) were just as likely due to environmental and behavioral factors as to the dictates of an intrinsic biological clock.8 Timing of meals, for instance, could alter glycogen stores. (It has been shown, in fact, that alterations in meal times can shift time of peak sprinting ability to later in the afternoon.) Some have claimed that coffee intake or, conversely, caffeine withdrawal can also affect periodicity of performance. Poorer performance in the morning might reflect sleep effects, particularly in joint stiffness. Other extrinsic factors to be considered are differing periods of precompetition rest, daily changes in ambient temperature, depressed morning levels of motivation, and scheduling of competitions.
Drust and colleagues directly rebutted these arguments, contending that abundant evidence indicates that diurnal periodicity in sports performance is an expression of endogenous biological rhythms. They note that many of the extrinsic factors postulated by Youngstedt and O’Connor, when cyclical changes of physiological function and performance persist, have been controlled in the laboratory setting.
To figure this out, it would be helpful to do a study to see if diurnal patterns of athletic performance continue to be evident in a constant routine condition (fixed light exposure, diet, and so on). Unfortunately, this would require playing soccer in a cave for 24 hours with a mandatory hourly intake of sports drink and chocolate bars. Therefore, such an investigation has not been done. The closest is the study noted previously by Reilly and Walsh of soccer competitors in constant play for four days and nights, while rhythms of activity, RPE, and heart rate persisted. (The difficulties of performing such studies is highlighted in this report. Meaningful activity was lost in the fourth day due to “recurring instances of behavioral abnormalities.”)
If you want to learn firsthand about circadian rhythms and performance, you should head to Fairbanks, Alaska, on June 21. Just tell your spouse, parents, or significant other that you’re going out for milk, and go.
June 21 st. In case you’ve forgotten, that’s the date of the summer solstice, when the sun annually reaches the highest point in its arc across the sky in the Northern Hemisphere. In Alaska, the solar globe dips below the horizon for a short time, but it remains light all night long. This celestial event has been marked by mystics for centuries. Every year in Fairbanks since 1906, they’ve done it by playing baseball in the Midnight Sun Game. The local semipro team, the Goldpanners, hosts a team from elsewhere in the lower 48 states. The game starts at 10:30 at night and finishes around 1:30 the next morning. They never use artificial lighting, and the game has never been called for darkness. At midnight the game stops, and somebody traditionally sings the Alaska flag song.
How does the fact that the players’ circadian clocks are a bit off-kilter affect the quality of the game? It’s hard to tell for sure, but in terms of pure excitement, these games have been hard to top (a nifty pun, considering the competition takes place a mere 160 miles [260 km] from the Arctic Circle). In fact, in three straight years between 2000 and 2002, the game was decided by the Goldpanners in their final time at bat.
For equal drama, take the 1965 game. (A newcomer named Tom Seaver had started on the mound for the Goldpanners but had suffered a cut while trying to make a bare-handed grab of a short hopper in the fifth inning. He had to spend the rest of the game in the hospital emergency room.) It was the ninth inning, the Goldpanners led 4-3, and the Trojans of the University of Southern California were up. The bases were loaded, two were out, and the count was 3-2 on the batter Ken Walker. Two foul balls. Then—crack!—a powerful drive towards the left field wall. Which, with (or thanks to) a relieved gasp from the 2,500 frenzied hometown fans, curved foul. Muggs Mies went into his stretch, delivered the decisive curve ball, and struck Walker out! One can only suspect a Hollywood scriptwriter.
Perhaps with Solomon-like wisdom, Drust and colleagues had the best answer:
“Instead of considering that the circadian rhythms of sports performance are wholly exogenous in origin, an alternative explanation would be to consider that the complex changes required for exercise have both exogenous and endogenous components, and that the endogenous component is a reflection of the body clock. This implies that the circadian response to exercise is similar in origin to the rhythm of core temperature, and this could account for the general parallelism between the two rhythms.”8
Just how important or useful to the competitive athlete is this whole concept of diurnal rhythmicity of physiological function and performance? I suppose it depends on how one looks at it. Certainly, documentation is clear that peak performance in many sports events involving muscular power occurs in the late afternoon and early evening. Other data suggest that when mental acumen and fine motor function are important, morning or early afternoon is best. The amplitude of swings in function over the course of a day are appreciable, and could reasonably be expected to bear importance for performance, particularly in sports events in which very minor differences separate out the winner from the also-ran.
Drust and colleagues thought this way. The current research data, they said, “makes an understanding of the circadian variation in sports performance an important practical consideration for both athletes and coaches in competition and might have important implications for both the short-and long-term success of an athlete or team.”8
On the other hand, a number of issues, considered collectively, suggests that all this information is, although very interesting, of limited practical value to the competing athlete.
1. Athletes and coaches rarely, if ever, have much of a say in the scheduling of athletic events. Spectator sports are arranged around times when people can come and watch, or are dictated by television scheduling. At the U.S. Open, depending on weather and duration of matches, tennis competition can begin early in the morning or much later, even approaching midnight.
2. Circadian rhythms affect all the competitors, not just you. Your opponents at the starting line are just as likely to gain from a 6:00 p.m. firing of the gun.
3. There is probably considerable variability among athletes for peak times of performance, which cannot be easily defined for any particular person.
4. Even if significant improvements in performance could be expected at certain times, environmental influences might well wash out the advantage. Like the wind picking up to 20 mph just before the afternoon javelin throw. I know if I need to be mentally sharp for a particular task, a tall latte will do it. Too, we’re all well aware of dramatic fluctuations in performance that seem to come out of the blue for no obvious reason. The qualifier who knocks out the defending Wimbledon champ in the first round. The football team, trailing 23-0 after a disastrous first half, that comes out of the locker room looking like another team and rallies to win. All of these influences might negate biological-based circadian variations in performance.
It would be of great interest to athletes, on the other hand, if circadian rhythms could be demonstrated for effects of sports training. Picking the best time of day to practice, with the expectation of greatest performance improvement—now, that would be useful. (Keep in mind that improvements in performance from training result from adaptations to the stresses imposed by an exercise bout. So, we could be talking about circadian rhythms of the level of stress imposed by an afternoon’s training run, or the magnitude of the body’s aerobic physiological responses.)
We have little data to go on to see if daily periodicity of training responses actually occurs. Torii and colleagues reported that increases in estimated O2max were greater when subjects trained for four weeks in the afternoon (between 3:00 and 3:30 p.m.), compared to those training in the morning or evening. (All subjects were tested in the afternoon.) Hill and colleagues reported that improvements in aerobic endurance time for a high-intensity work load (2.6 W/kg) in the morning was greater than in the afternoon in a small group of subjects (N = 6) who trained in morning. The opposite was true for those who trained in the afternoon. This suggested that training-induced performance might be predicted to occur optimally at the same time of day as the training. In another study, however, Hill and colleagues were unable to detect any circadian variations to training-induced improvements in O2max. They did show, though, that for subjects who trained in the morning, values of ventilatory thresholds were higher after training. Those who trained in the afternoon demonstrated greater ventilatory thresholds when tested in the afternoon.9
Hildebrandt and colleagues described a 20% greater increase in muscle strength when subjects trained at 7:00 p.m. than at 9:00 a.m. In that study, as Reilly and colleagues pointed out, subjects were only tested at the time they trained.
These meager data provide just an inkling that there might be something to this idea of a best time of day for optimizing training effects. Athletes would be most interested if there were. Here’s something concrete, like diet and equipment, that they could readily control. Clearly, a great deal of research work needs to be done before any conclusions on the matter can be drawn.
So far, we’ve been considering rhythmic changes in physiological factors and sports performance that vary in a circadian, or diurnal, fashion. What about patterns of change that fluctuate over a year’s time, or with the seasons? Certainly, many examples of biological circannual rhythms exist among animals (hibernation, mating, migration). In humans, biological and environmental factors may combine to cause seasonal patterns in myocardial infarctions, asthma, mood, weekly energy expenditure, and body composition. Do these kinds of longer phasic changes occur in sports performance?
You can already see the problem in trying to answer this question. Many sports are performed only at particular times of the year, either by dictates of climate or custom, which presumably have nothing to do with intrinsic biological clocks. [A researcher from outer space landing on Earth might well conclude that annual biological rhythms must exist for football (coincidentally and maybe causally related to the falling of leaves), baseball (with a peak in the summer corresponding to beer consumption), and skiing (peak at times of children’s winter school vacations).] Even in sports that are performed year-round, training regimens are periodized by athletes in order to peak for major competitions. These extrinsic factors are just too prominent to permit insights into any true biological circannual rhythms.
That’s what Atkinson and Drust concluded in their review of seasonal rhythms and exercise.10 They cite a couple of studies. Members of a British road-cycling team had a higher O2max during a competitive season. Elite-level Dutch speed skaters demonstrated no seasonal changes in O2max or performance on a Wingate anaerobic test. The bottom line is that seasonal changes in sports performance can usually be explained by extrinsic patterns of climate or training intensity. At present, no evidence exists of any meaningful influences of intrinsic circannual rhythms on sports performance.
The evidence of the traditional argument for the strength of intrinsic biological rhythms becomes painfully clear when such rhythms are disturbed. This happens when you are transported rapidly over multiple time zones on the planet. Circadian rhythms are entrained principally to 24-hour cycles of light and dark. If your 747 takes off from JFK in New York at 8:00 on a winter evening, your biological clock (with its created physiological rhythms) is expecting that the dark portion of the cycle will be completed in about 12 hours. To its great surprise, however, it instead encounters light just six hours later as you touch down at Charles de Gaulle airport outside of Paris.
Such disturbances of circadian rhythms produce what we know, of course, as jet lag, that temporary but particularly unpleasant set of sensations that are experienced by almost all international travelers. Its symptoms include fatigue, poor sleep, decline in mental acumen, gastrointestinal upset, irritability (particularly, from personal experience, with one’s spouse and children), headaches, and even mental confusion. Not exactly the way you’d want to spend your first day in Paris.
Although its mechanism remains to be clarified, there are some really interesting things about jet lag. Symptoms are worse when you’re flying from west to east than the reverse. The more time zones you cross, the worse you’ll feel. It is said that for each time zone you cross, you should add another day to fully recover. If you’re young (pick your own definition) and in good physical shape, the effects will be less. In general, females probably experience more jet lag than males. Personality types seem to influence the magnitude of jet lag symptoms. If you’re an extrovert or a night person, for example, you are likely to adapt more quickly to perturbations in biological rhythms. If you’re highly neurotic, be prepared to suffer more. And if you stay in your hotel room after arriving rather than touring Notre Dame, the duration of your symptoms may well be extended.
It’s not unreasonable to expect that jet lag with overseas travel would impair both physical and mental performance in a competitive sports situation. Athletes participating in international events have thus had great interest in the possible negative effects on physiology and performance of what has been termed circadian adversity. More importantly, they’d like to know how to avoid them. Surprisingly, the research literature regarding the effects of jet lag for athletic performance is not considerable. Among the few studies that have been conducted, many suffer from significant weaknesses in design. Consequently, uncertainty persists about the effects that disturbed biological circadian rhythms, per se, have on sports performance following international travel. Here is some of the research information we do have.
Wright and colleagues described what happened to physiological function and exercise performance of 97 male soldiers when they were transferred from Fort Hood, Texas, to a military base in Germany, a translocation of six time zones. Testing was performed during five days prior to their departure and repeated during the first five days of arrival, beginning eight hours after their plane landed. A majority of the soldiers described typical jet-lag symptoms of fatigue, weakness, headache, disturbed sleep, and irritability. These substantially diminished by the fifth day. Despite these complaints, maximal oxygen uptake, running economy, and RPE during treadmill testing were unchanged, as were measures of isometric strength. However, a decline in dynamic strength and muscular endurance of elbow flexors was observed. Similarly, performance on a 270-meter sprint deteriorated between 8 and 12% (figure 4.3).
Hill and colleagues reported that peak anaerobic power and 30-second anaerobic work capacity on a Wingate cycling test were reduced when a group of nonathletic subjects traveled from North America to France. The trip failed to affect grip strength, however.
A number of studies have attempted to assess differences in performance of athletic teams after long-distance travel to competitions. For example, among 8,495 games in the National Basketball Association, visiting teams that traveled from the west coast to the east coast of the United States were found to have scored four points a game better than teams flying in the opposite direction. (However, other explanations for this observation might exist rather than the directional differences in jet lag. Perhaps the teams on the west coast are simply better than those from the east.) In another study, win-loss records were examined for National Football League teams between 1978 and 1987. It was found that the home-field advantage rose with increasing distance traveled by the visiting team.
Although it is generally assumed that long-distance travel across numerous time zones will result in a deterioration of athletic performance on arrival, a number of authors have not been convinced. On reviewing the experimental data, O’Connor and Morgan concluded that “no evidence exists in support of the view that rapid traversal of multiple time zones has an influence on athletic performance.”11 Youngstedt and O’Connor agreed, claiming that “the scientific evidence supporting the assumption [that air travel has detrimental effects on athletic performance] is neither consistent nor compelling.”8 Even if such decrements were to be documented, these detractors note that other factors during long-distance travel, such as disturbances in sleep cycles, fatigue, boredom, dehydration, and change in dietary patterns, might be just as likely to cause declines in performance as the desynchronization of biological circadian rhythms.
Equally uncertain is the efficacy of the many preventive measures and remedies that have been utilized by international travelers in the hope of avoiding jet lag. These have included dietary intake plans (high-protein breakfasts, high-carbohydrate dinners), gradually changing sleep patterns before traveling, exercise, light exposure, hydration, sleeping pills, alcohol, and supplemental melatonin. And if the American athlete is flying to Singapore, the advice would be to go through Hawaii rather than London (travelling from east to west is best). The most sound advice, however, remains to fly early enough to permit dissipation of jet-lag symptoms before the start of competition.
The preceding discussion has focused on what happens when you cross time zones and make large hops of longitude by traveling east to west or vice versa. But what happens if you take a long trip north or south, crossing latitude lines, maybe without even leaving the same time zone? This, in fact, might be expected to disturb circadian rhythms in another manner. Diurnal biological rhythms are entrained to light-dark cycles, but such phases of light and dark vary dramatically as you move from the equator toward the poles. If you fly on June 21 from Central America, where the light-dark ratio during 24 hours is about 12:12, to Greenland, you will find the ratio has changed to almost 24:0.
The circadian clock adjusts for such changes in the light-dark cycle (as it must do during the changes of seasons throughout the year in a single location). You might expect, however, that for a brief period after you land, your physiological rhythms will be in a bit of disarray and your capacity for physical performance might suffer. The effect of sudden marked latitudinal displacement on athletic performance, however, has not been determined.
No discussion of circadian rhythms, jet lag, and sports performance would be complete without a mention of melatonin, which may serve as the mediator by which the biological clock in the suprachiasmatic nucleus (SCN) is linked to light-dark cycles. Melatonin is secreted by the pineal gland in a diurnal fashion that is very strongly influenced by light. Levels during the dark of night can be 30 times greater than in the daytime. And the SCN is equipped with melatonin receptors, presumably to stay in tune with light-dark cycles.
What happens to blood-melatonin levels when you exercise? The reports have been conflicting. One study found that that plasma-melatonin levels in adult females rose transiently at least twofold after 60 minutes of exercise. This is similar to previous findings in men. But others have described either a decline in melatonin concentration 3 hours after 20 minutes of exercise, or no changes in blood-melatonin levels at all after sessions of exercise.
It should be recognized that these reports describe different forms of exercise performed at various times in the day-night cycle, with levels drawn at different times postexercise. The true response of plasma-melatonin levels to physical exercise remains to be clarified.
On the other hand, melatonin has a number of physiological effects that seem rather incontrovertible (and of some interest to athletes), including the following:
• Lower body temperature (hypothermic effect)
• A sedative or hypnotic effect
• Alleviation of symptoms of jet lag, perhaps by speeding the resetting of circadian rhythms after long-distance travel (This effect is somewhat less definite.)
From these influences, one might immediately suspect that administration of melatonin might offer some real advantages to the athlete in promoting exercise performance. For instance, a reduction in body temperature would be expected to permit greater aerobic endurance when competing in hot ambient conditions. And alleviation of jetleg symptoms could reduce any decline in motor performance after international travel. But, alas, the experimental data addressing these issues is disappointing. It doesn’t seem to work, at least in respect to reduction of hyperthermia with exercise. And taking melatonin before some kinds of athletic events might even be detrimental to performance.
Atkinson and colleagues provided a table outlining 17 studies that have demonstrated a hypothermic effect of administered melatonin, with a reduction in core body temperature ranging from 0.01 to 0.3 degrees C.12 Interestingly, although the diurnal periodicity of melatonin secretion from the pineal gland (highest in the night) is the mirror image of that of core temperature, no one has been able to verify a causal relationship between the two. This hypothermic effect of melatonin, however, has not been observed to translate into any improvement of exercise tolerance in hot ambient conditions.
In its hypnotic qualities, melatonin has been documented to slow the subject’s visual reaction time as well as reduce alertness. But if you are a rat, here’s good news. Melatonin can be expected to improve your short-term memory (exactly for what is uncertain). Unfortunately, if you are a human, no similar effect has been observed. In fact, in performance tests, melatonin has been observed to decrease short-term memory, as well as balance and proprioception. The influences would not seem to bode well for certain kinds of sports performance. Atkinson and colleagues noted, though, that if you were to take melatonin in the evening to promote sleep, any negative effects on cognitive performance should disappear by the following morning. The bottom line is, though, that despite certain hypothetical advantages, there is no reason to expect that administration of melatonin for its hypothermic or hypnotic effects will enhance athletic performance.
Melatonin may have a role in preventing jet lag, however. If these symptoms of travel-induced disturbances of circadian rhythms truly do deteriorate sports performance, it is a possible remedy. A number of extensive reviews have examined the published research literature regarding the ability of melatonin to reduce jet lag. The general conclusion is that it often works, at least to some extent. Just how is unknown. It could be a result of its sedative effect or it might be a matter of shifting circadian rhythms.
Would taking melatonin help an athlete flying over the Atlantic Ocean to participate in a track and field meet in London prevent symptoms of jet lag, blocking any impairment of performance the next day? That’s a key question. The simple answer is that no one knows. To my knowledge, the question has never been addressed experimentally. But there is one report in soldiers, who serve as a reasonable surrogate.
Lagarde and colleagues assessed changes in both static and dynamic exercise performance for U.S. Air Force reservists on a flight from San Antonio, Texas, to an air base in Landes, France. Nine subjects took 5 milligrams of melatonin one day before the flight, again during the flight, and also during the first three days after landing. Another group took a placebo. On testing hand-grip strength before and after travel, the placebo group demonstrated a decrement in performance during the initial three days in France, while no decline was seen in those taking melatonin. The trip did not affect performance of either squat jumps or multiple jumps in either group. So, in this study, melatonin seemed to reduce the negative effect of jet lag on static movement, but not dynamic exercise.
Despite its potential usefulness, there seems to be general cautiousness about taking melatonin to alleviate jet lag symptoms. Proper timing and dosage have not been completely determined. Although taking melatonin has generally been considered safe, without acute side effects, long-term toxicology studies have not been performed. For those interested in preventing decrements in sports performance after long-distance travel, such discretion may be particularly warranted.
So what does all this mean for the athlete traveling long distances for international competition? In 2004, having reviewed most of the material you’ve just read, the FIMS (Federation Internationale de Médecine du Sport) issued an official position statement titled “Air Travel and Performance in Sports.” Their conclusion? “Although some athletes, anecdotally, report impaired performance after air travel, there is no consistent or compelling evidence showing that either air travel across multiple time zones or jet lag symptoms cause a reduction in sports performance.”13
Just in case you’re one of those anecdotal few, however, the document outlines a number of practical strategies that athletes might consider in order to alleviate or prevent symptoms of jet lag. These include exposure to bright light (which reportedly creates shifts of biological phases), melatonin (which shifts the biological clock in the opposite direction as bright light), and exercise (which acts like melatonin). Refer to this source for the details, which are numerous, as to the most effective timing of these interventions (www.fims.org). Be careful, though. If you err on the proper timing, you might only make things worse.
So notes the FIMS disclaimer: “It cannot be overemphasized that these three methods for shifting circadian rhythms depend critically on the timing of the intervention. Ill-timed bright light, exercise, or melatonin could plausibly exacerbate jet-lag symptoms or delay resynchronization beyond the time it usually takes to adapt to a new time zone.” I was not able to find in this position statement, though, the seemingly best advice: Arrive at the destination early enough to allow symptoms of jet lag to subside before the day of competition.
1. Reiberg, A.E. et al.2001. “The birth of chronobiology: Julien Joseph Virey 1814.” Chronobiology International 18: 173-186.
2. Read a fascinating first-person account of his two-month subterranean ordeal: Siffre, Michel. 1964. Beyond time. New York: McGraw Hill.
3. This book provides an impressive listing of just how markers of body function, disease, and biochemistry in the human body ebb and flow over the course of a day: Foster, R.G., and L. Kreitzman. 2004. Rhythm of life. New Haven, CT: Yale University Press.
4. This book is an excellent source of information on circadian rhythms: Dulap, C., J.J. Loros, and P.J. DeCoursey. 2004. Chronobiology. Biological Timekeeping. Sunderland, MA: Sinauer Associates.
5. Reilly, T., G. Atkinson, W. Gregson, J. Forsyth, B. Edwards, and J. Waterhouse. 2006. “Some chronological considerations related to physical exercise.” Clinical Therapy 157: 249-264.
6. For more information on the effect of circadian rhythms on physiological variables, read the following sources: Deschennes, M.R., R.V. Sharma, K.T. Brittingham, D.J. Casa, L.E. Armstrong, and C.M. Marsh. 1998. “Chronobiological effects on exercise performance and selected physiological responses.” European Journal of Applied Physiology 77: 249-256. Nicolas, A., A. Gauthier, N. Bessot, S. Moussay, and D. Davenne. 2005. “Time-of-day effects on myoelectric and mechanical properties of muscles during maximal and prolonged isokinetic exercise.” Chronobiology International 22: 997-1011.
7. Reilly, T, and T. Walsh. 1981. “Physiological, psychological, and performance measures during an endurance record for 5-a-side soccer play.” British Journal of Sports Medicine 15: 122-128.
8. Conflicting viewpoints of the effect of circadian rhythms on athletes can be found in the following: Drust B., J. Waterhouse, G. Atkinson, B. Edwards, and T. Reilly. 2005. “Circadian rhythms and sports performance—an update.” Chronobiology International 22: 21-44. Youngstedt, S.D., and P.J. O’Conner. 1999. “The influence of air travel on athletic performance.” Sports Medicine 28: 197-207.
9. References for circadian rhythm effects on training responses: Hill, D.W., J.A. Leiferman, N.A. Lynch, B.S. Dangelmaier, and S.E. Burt. 1998. “Temporal specificity in adaptation to high intensity exercise.” Medicine and Science in Sports and Exercise 30: 450-455. Hildebrandt, G., C. Guttenbruner, C. Reinhart et al. 1990. “Circadian variation of isometric training in man.” In Chronobiology and chronomedicine Vol. II, ed. E. Moran E, 322-329. Frankfurt: Peter Lang. Torii, J., S. Shinkai, S. Hino et al. 1993. “Effect of time of day on adaptive response to a 4-week aerobic exercise training program.” Journal of Sports Medicine and Physical Fitness 32: 348-352.
10. Atkinson, G., and B. Drust. 2005. “Seasonal rhythms and exercise.” Clinics in Sports Medicine 24: e25-e34.
11. Additional reading on jet lag and athletic performance: O’Connor, P.J., and W.P. Morgan. 1990. “Athletic performance following rapid traversal of multiple time zones.” Sports Medicine 10: 20-30.
12. Atkinson, G., B. Drust, T. Reilly, and J. Waterhouse. 2003. “The relevance of melatonin to sports medicine and science.” Sports Medicine 33: 809-831.
13. O’Connor, P.J., S.D. Youngstedt, O.M. Buxton, and J. Breus. 2004. “FIMS Position statement.” Air Travel and Performance in Sports. www.fims.org.
1. Regular phasic changes in biological function are an inherent quality of all living beings. These oscillations, which are usually diurnal or circadian, include variables that influence athletic performance.
2. The extent that circadian rhythms in physiological and psychological variables truly affect athletic performance and responses to training remains uncertain. Particularly, it is not clear whether the magnitude of such effects outweighs those of extrinsic or environmental factors.
3. Although they are not well documented, it is reasonable to expect that interruptions in circadian rhythms, as manifested by symptoms of jet lag, may transiently negatively affect athletic performance after long-distance travel.
