No matter how much a person loves eating or sex, one could never spend as much time doing them as sleeping. An average person may sleep 200,000 hours in her lifetime. What is the function of sleep? If sleep is a form of cessation of activity like catching your breath, there is no reason to search for neural mechanisms responsible for sleep. However, if sleep is a special state of consciousness serving a particular function, then researchers should seek answers as to how it is regulated.
The environment in which you live is full of routines and cycles. Of course the most common cycle is the day-night cycle that occurs over every twenty-four hours. Most of the animals that are exposed to this light-dark cycle have adapted to this timed routine such that they have fairly regular schedules of sleeping, waking, eating, etc. Circadian rhythms are these daily cyclical adaptations, which can be biological or functional in nature.
The most obvious circadian rhythm is the sleep-wake cycle that people and animals adhere to; however, many other biological processes, such as body temperature and hormonal levels, adjust themselves to a circadian rhythm. Moreover, circadian rhythms such as the sleep-wake cycle still endure in conditions where there is no light-dark cue. In laboratory conditions when animals or people live in total dark or total light, it appears that the cycle adjusts to regularities in the animal’s environment such as regular meal times, regular social interactions, and other events that are predictable.
When a person works on the night shift, he tends to sleep during the day. Daytime sleep periods are about an hour to two hours shorter than nocturnal sleep periods. Stage II sleep and REM sleep are typically more affected than other stages, and this leads to problems with alertness and fatigue. It is not uncommon for shift workers to become obsessed with sleep.
It also appears that the circadian rhythm can deviate from the twenty-four-hour cycle. Animals exposed to eleven-and-a-half-hour cycles of light and dark will display a circadian rhythm that adjusts to a twenty-three-hour cycle. When environments are devoid of events that can serve as cues for circadian rhythms, it appears that these rhythms are still maintained. These free-running rhythms, as they are termed, have free-running periods that vary in length and are only slightly longer than twenty-four hours, suggesting that people have an internal biological clock that approximates the twenty-four-hour cycle.
An internal circadian clock is the biological timing mechanism that is responsible for the circadian rhythms in people and animals. The area of the brain that has been linked to circadian cycles is called the suprachiasmatic nucleus. The suprachiasmatic nucleus is located in the medial area of the hypothalamus on both sides of the brain, opposite the optic chiasma, which is the point where the optic nerves cross over to the opposite hemisphere of the brain. The suprachiasmatic nuclei appear to be involved in the setting of light-dark cycles of circadian rhythms, by way of neural projections from the optic nerves. It appears that axons project from the optic nerve into the area of the optic chiasma and project to the suprachiasmatic nuclei. These projections have been termed retinohypothalamic tracts; therefore, visual information is the cuing mechanism for this route, and the suprachiasmatic nucleus appears only to be involved in circadian rhythms that are cued by light-dark cycles, not those triggered by other stimuli. When animals in experimental conditions have this area removed or lesioned, other environmental cues, such as food, can still trigger circadian rhythms, but light-dark cycles do not. So while the suprachiasmatic nuclei do appear to be involved in the light-dark circadian rhythms, there obviously are other areas of the brain that are also involved in regulating these cycles.
Biorhythms are attempts to track various aspects of functioning—emotional, intellectual, and physical cycles—using mathematical techniques. Empirical research has indicated that these techniques and their assumptions are not reliable, and most scientists find no support for them. Chronobiology is the study of biological cycles, such as the circadian rhythm.
One mechanism that may contribute to circadian rhythms is genetics. Certain genes are involved in circadian rhythms, and many of the same gene types have been identified in many different species of animals. More recent research has indicated that molecular circadian timing mechanisms, similar to the type in the suprachiasmatic nucleus, exist in many cells of the body, and that these are affected by certain hormones and neural influences from the suprachiasmatic nucleus.
The amount of sleep that people get is variable from one person to the next, and the common prescription is everyone should get eight hours a night. Most people who live to be seventy-five years old will have spent almost twenty-five years sleeping. There are many changes that occur during sleep, and most of the studies that investigate the processes occurring during sleep use either EEGs (that measure brainwave activity), electro-oculograms (EOG, that measure retinal activity), or electromyograms (EMG, that measure electrical impulses of muscles) to record their findings. EEG findings have determined that there are four separate stages of sleep that are appropriately named stage I, stage II, stage III, and stage IV. There is also another stage, REM sleep.
Before you fall asleep, there is a period of relaxed wakefulness that produces EEG waves known as alpha waves that have a frequency of about 8 to 12 waves per second. Alpha waves are characteristic of a relaxed state. Once one enters stage I sleep, one produces EEG waves that are irregular, jagged, and low-voltage, indicating that brain activity is declining. These waves are similar to those produced during alert wakefulness but are much slower.
The most prominent EEG characteristics of stage II sleep are sleep spindles and K-complexes. A sleep spindle is a burst of 12 to 14 Hz waves that last at least a half second (typically these last for two or three seconds). The K-complex consists of a sharp, high amplitude, negative wave followed by a smaller, slower, positive wave. K-complexes are most common during stage II sleep, but sudden stimuli can evoke these in other stages of sleep.
Stage III sleep reveals the presence of delta waves, which are the largest and slowest brain waves recorded on an EEG. Delta waves are also predominant in stage IV sleep, thus leading to stages III and IV sometimes being called slow-wave sleep.
As you fall asleep and move through the stages, your breathing rate, heart rate, and brain activity become slower than they were in the previous stage, and the percentage of slow, large amplitude waves increases. These slow brain waves indicate that neural activity is becoming highly synchronized, whereas during wakeful states, neural activity is a less synchronized response to a number of external and internal stimuli. Once at stage IV sleep, you remain there for a while and then cycle back through stages III and II, but instead of going to stage I, you enter a new stage called REM sleep (rapid eye movement sleep). Some researchers prefer to distinguish only between REM sleep and non-REM sleep (all of the other stages).
REM sleep is obviously associated with rapid eye movements, but also with a loss of muscle tone in the body’s core and high-frequency, but low amplitude, EEG waves. Cerebral activity increases such that REM sleep is similar to waking states. Autonomic nervous system activity also waxes and wanes during REM sleep. There can also be occasional twitches in the extremities during REM sleep and in many males there is penile erection. REM is associated with dreaming, and for many years some texts suggested that dreaming only occurs during REM sleep; however, even early studies of dreaming and REM sleep indicated that people could remember dreams from non-REM sleep, even if this was infrequently. So while it appears that a great deal of dreaming does occur during REM sleep, it is also certain that some dreaming occurs in other stages.
The hypothalamus is one of the main areas of the brain that appears to be involved in sleep. Very early discoveries based on clinical data indicated that people who had damage to the posterior hypothalamus and adjacent areas experienced problems with excessive sleep. Other studies of individuals who had damage to the anterior hypothalamus experienced difficulty sleeping; thus, early findings based on clinical studies of patients suggested that the anterior hypothalamus is involved in promoting sleep and the posterior hypothalamus is involved in wakefulness.
The sleep-wake cycle appears to be a function of an area of the brain called the reticular activating system or reticular formation. The reticular formation is an extensive network of nuclei and nerve pathways located throughout the brain stem. This series of structures connects both motor nerves and sensory nerves to and from the spinal cord and the cerebellum, and the reticular formation also has widespread connections to the cerebrum. The neurons in the reticular formation also have extensive connections with other areas of the brain and perform numerous other functions. It has been estimated that a single neuron in the reticular formation may have connections with as many as 25,000 other neurons. This important center of the brain is involved in alertness, sleeping, attention (for example, filtering out stimuli that are unimportant and selecting stimuli that are important for further scrutiny), pain control, cardiac functions, and other functions.
The reticular formation is involved in alertness and the ability to filter out irrelevant stimuli. The reticular formation also produces acetylcholine, an important neurotransmitter. Bilateral damage to this area may result in coma or death. Neuroimaging studies have shown abnormal functioning in the reticular formation in people with chronic fatigue syndrome, suggesting that this area has some interplay with the fatigue associated with certain disorders.
The posterior portion of the reticular formation appears to be involved in the production of REM sleep. There are several sites at the posterior portion of the reticular formation that each appear to be involved in a separate function relating to the characteristics of REM sleep. One site of the posterior reticular formation appears to be involved in rapid eye movements, another site in the reduction of muscle tone, and so forth.
Dreams have intrigued people for centuries. There are numerous stories of dreams having the ability to predict the future or to have some deep hidden meaning. Freud believed that dreams were expressions of a person’s unconscious needs and drives, and he used dream interpretation as a part of his psychoanalysis. Nonetheless, there is no evidence that dreams are premonitions or that the Freudian theory of dreams is valid. More current theories, such as the activation-synthesis theory, suggest that dreams result from a mass of information supplied to the cortex during REM sleep and that this information is random. The dream is an attempt by the brain (cortex) to make sense of these signals.
Some people claim that they never dream; however, in clinical studies these people have as much REM sleep as other people who do recall dreams, and if they are awakened during REM sleep, they do report the aspects of dreams. Another myth about dreams is that dreams only last a few seconds or less, but again clinical research has refuted this belief. The research suggests that dreams run on real-time, so if your dream feels like it lasted for thirty seconds, it probably lasted pretty near thirty seconds.
Sleepwalking (somnambulism) does not occur during dreaming, as the muscles in one’s core are totally flaccid during REM sleep. Sleepwalking usually occurs during stage III or stage IV sleep. Talking in one’s sleep has no relation to REM sleep and can occur at any stage of sleep, but most often occurs right before awakening.
More evidence that the cortex tries to make sense of the incoming stimuli it receives during a dream can be seen from studies that test the effects of external stimuli on people in REM sleep. Many times, but not always, the external stimuli become incorporated into the dream (thus, a loud noise occurring during REM sleep can be incorporated into a dream as an explosion or similar event). The penile erections that occur in males during dreams are often assumed to be associated with sexually explicit dreams; however, even male babies have REM-related erections. Other research has indicated that erections are no more common during sexual dreams than other dreams in males.
Why do animals sleep? Believe it or not, this is a million-dollar question. For many people the function of sleep is obvious: to get rest. This argument is the essence of one of the major theories on why animals sleep: the recuperation theory of sleep. The recuperation theory proposes that being awake for long periods of time disrupts the homeostasis of the body and that sleep restores homeostasis. There are a number of different theories that qualify as recuperation theories, and many single out different functions that sleep restores, such as energy restoration, reparation of the body tissues, and others. According to most of these theories, the period of sleeping naturally ends when homeostasis is restored to some extent.
The other side of the coin is the idea that sleep is not a reaction to the detrimental effects of being awake; it is a result of an internal twenty-four-hour timing mechanism that animals and humans are programmed to follow. According to these adaptation theories of sleep, people have evolved to sleep at night because it protects them from predation or misfortunes that could happen in the dark. Thus, sleep has a protective function. These theories tend to focus more on the timing of sleep as opposed to the function of sleep, and some variations of these theories hypothesize that sleep has little restorative function. For these theorists, sleep is like sex in that people are motivated to do it, but it is not essential to individual health.
There is quite a bit of variation in the amount of time that mammals sleep. A horse may sleep as little as two hours a day, whereas house cats may sleep fourteen hours a day. The giant sloth may sleep as much as twenty hours a day, but that is still not as much as koala bears, which may sleep as much as twenty-two hours a day.
The interesting thing about sleep is that nearly every species of mammal and bird sleeps, and there is evidence that other species sleep, such as reptiles, fish, and insects. This suggests that sleep does serve some important biological function and that adaptation theories of sleep are probably not valid. Moreover, the fact that higher species of animals sleep indicates that the function of sleep, whatever it is, is not restricted to people alone. However, whatever this biological function is has yet to be determined.
There are many puzzling and inconsistent facts about the sleep patterns of animals. There does not seem to be a relation between how many hours a particular species generally sleeps and its size, activity level, or other important features that would indicate restorative functions of sleep. One thing that does seem to make sense is that there is a relation between the daily sleep time of the particular species and its vulnerability to predation. This is more consistent with adaptation theories of sleep. For example, many predators such as lions sleep many hours a day, whereas their prey (often grazers who spend a lot of their time eating) sleep only two to four hours a day. Yet other animals with slow metabolisms like sloths sleep up to twenty hours a day, counterintuitive to the notion that sleep performs a restorative function. So the actual function of sleep remains somewhat of a mystery to researchers.
One way to ascertain the function of sleep is to look at the research on sleep deprivation. There have been quite a number of studies that have looked at sleep deprivation in human participants. Unfortunately, many of these studies do not control for stress. For instance, most people feel the effects of sleep deprivation during stressful periods in their lives, such as an emergency, work or school issues, changes in schedule, etc. The changes in functioning that people often feel in these instances are confounded by stressful events, as many of the reported changes are similar to the effects of extreme stress. Thus, many of the popular studies of the effects of sleep deprivation are confounded by the effects of stress.
The recuperation theories of sleep would predict that when one is deprived of sleep, this would lead to behavioral and physiological problems, that these problems would worsen as the period of deprivation continues, and that once one does sleep, the amount of missed sleep would be regained.
The results of most of the sleep deprivation research indicates that even mild amounts of sleep deprivation will lead to increases in sleepiness, poorer performance on sustained attention tasks, and some negative cognitive and mood effects. However, the relationship between sleep deprivation and cognitive skills appears to be complex.
First, a substantial amount of sleep deprivation appears to be required to produce consistent alterations of the results on cognitive tests. Secondly, sleep deprivation appears to affect only specific cognitive functions. For example, abilities that are relatively well learned may not be affected. Research investigating the effect of sleep deprivation on logical deduction or on the ability to think critically has also indicated that sleep deprivation does not affect these abilities. However, tests of executive functions (e.g., assimilating new information into strategies and plans, switching goals, and more abstract types of thinking) appear to be susceptible to the effects of sleep deprivation.
There has recently been a debate regarding sleep deprivation as a form of torture. Sleep deprivation was used as an interrogation method in wartime settings and recently with terrorist suspects. It is considered by some proponents as a “stress and duress” technique and is legal during interrogations. According to the United Nations, sleep deprivation is a form of torture.
In the overall analysis, the research on the effects of sleep deprivation on one’s cognitive abilities has been quite inconsistent and does not fully support the recuperation theories of sleep. Studies investigating the effects of sleep on the immune system have also delivered variable results.
A period of sleep deprivation alters the sleep patterns of the sleep deprived person for a short period. Following sleep deprivation, a person’s sleep becomes more efficient in that individuals have a higher proportion of stage III and stage IV sleep, both of which are hypothesized to serve the main restorative functions. In addition, people who sleep six or fewer hours per night on a regular basis appear to have the same amount of slow-wave sleep (stages III and IV) as people who get eight hours of sleep per night.
A number of studies have investigated REM-sleep deprivation where only REM-sleep periods are interrupted. During these studies, when an individual begins to transition into REM sleep, she is awakened and then allowed to go back to sleep. When this research is undertaken, the participants typically display what is termed a “REM rebound”; they experience more REM sleep than usual for the first several nights. This tendency of the system of a person deprived of REM sleep to try to compensate for decreases in REM sleep indicates that REM sleep may be regulated differently by the brain than are the other stages of sleep. It is also clear from these studies that REM sleep must be performing some very important functions.
One hypothesized function of REM sleep is the consolidation of memories. However, the research does not always support this. For example, many people who use antidepressant drugs that block REM sleep do not experience memory issues. Other studies have indicated that when deprived of REM sleep, people begin to experience hallucinations and other issues that resemble certain psychiatric disorders. However, these findings are not always replicated.
Sleep disorders are categorized into either insomnia or hypersomnia. Insomnia would include any disorder that consists of difficulties falling asleep or staying asleep. Hypersomnia would include any disorder that is characterized by excessive sleeping or sleepiness. Insomnia and hypersomnia are also symptoms of many different psychiatric disorders, such as depression, bipolar disorder, anxiety disorders, and others. Some sleep researchers would include a third class of sleep disorders that consists of any issues related to REM-sleep problems or dysfunctions.
In the general population, about 30 percent of people responding to surveys regarding their sleeping habits report that they have significant sleep-related problems. Many of these reports can be tied to misunderstandings about sleep. For example, many people believe that they must get at least eight hours of sleep a night but naturally function on less than that. Thus, these people are convinced that they have a significant problem with their sleeping habits. Other people may experience problems with sleep related to medication usage or excessive usage of caffeinated beverages, such as coffee, tea, soda, etc. In many of these cases, the anxiety that people feel regarding their “issues” with sleep makes it even more difficult for them to sleep properly.
Iatrogenic causes of insomnia are not uncommon (iatrogenic means “created by a physician”). Most often this type of insomnia is a result of medications or the side effects of medications. For instance, the use of sleeping pills, such as benzodiazepines, which are prescribed by a physician, is actually a major cause of insomnia because tolerance to these drugs is developed quite quickly. Once a person develops tolerance, more and more of the drug is required to get the same effect, and patients often begin experiencing withdrawal symptoms of which insomnia is but one.
It is quite common for individuals who claim to have problems sleeping to be overstating their problem. For instance, many people claim it takes them an hour or more to fall asleep, but when studied in a sleep lab, they typically fall asleep within fifteen minutes or less. Years ago when Freudian thought dominated psychiatry, many of these individuals were diagnosed with a neurosis, which is a psychiatric disorder that does not include severe psychosis, such as hallucinations. For many people with insomnia, an effective treatment is sleep restriction therapy. Initially, this treatment restricts the amount of time a person can spend in bed, and as time goes on and the person is able to sleep, he is allowed to spend more time in bed as long as he remains sleeping.
Sleep apnea occurs when the person stops breathing multiple times during the night. Each time this happens, the person wakes, breathes, and goes back to sleep. This routine can happen many times at night without the individual being aware of it. Individuals with sleep apnea often complain of being sleepy during the day. There are two types of sleep apnea: (A) obstructive sleep apnea, due to some obstruction in the respiratory passages, which often occurs in individuals who snore quite loudly, and (B) central sleep apnea, which occurs as a result of the CNS failing to stimulate respiration during sleep. Risk factors for sleep apnea include being male, snoring, being grossly overweight, and increased age.
Restless legs syndrome occurs when an individual experiences twitching or tension in his legs that keeps him from sleeping at night. Periodic limb movement disorder involves the involuntary movements of the limbs, often in the legs, which occur during sleep.
Narcolepsy consists of severe daytime sleepiness that usually occurs at inappropriate times. These people can fall asleep right in the middle of a conversation, while they are driving, and even during sex. People with narcolepsy also experience drop attacks or cataplexy, which is a recurrent loss of muscle tone while awake. These attacks may force a person to sit down, or the person may drop as if shot by a gun and remain down but fully conscious. Drop attacks are typically triggered by emotionally charged events. People with narcolepsy may also experience hypnagogic hallucinations, which are dreamlike experiences while awake, and sleep paralysis, which is the inability to move just as one falls asleep (many people without sleep disorders occasionally experience these). Narcolepsy results from disruptions in the mechanisms that trigger REM sleep and has been linked to reduced levels of a neuropeptide in the cerebral spinal fluid called orexin. Orexin is synthesized in the posterior hypothalamus (which has been linked to states of wakefulness).
Stimulant medications are the most common form of treatment for narcolepsy; however, these often have many side effects. Other treatments that are often used for narcoleptic patients are antidepressant medications, because these medications often suppress REM sleep in people who take them. Other forms of treatment are being investigated.