Learning Goals
After Chapter 4.3, you will be able to:
- Identify the two hormones most associated with circadian rhythms
- Distinguish between dyssomnia and parasomnia
- Associate the stages of sleep with their EEG waveforms and other main features:
After Chapter 4.3, you will be able to:
Consciousness is one’s level of awareness of both the world and one’s own existence within that world.
The accepted states of consciousness are alertness, sleep, dreaming, and altered states of consciousness. Technically, sleep and dreaming are also considered altered states, but we will consider these states separately from hypnosis, meditation, and drug-induced altered states of consciousness. Altered states of consciousness may also result from sickness, dementia, delirium, and coma.
Alertness is a state of consciousness in which we are awake and able to think. In this state, we are able to perceive, process, access information, and express that information verbally. In the alert state, we also experience a certain level of physiological arousal. Cortisol levels tend to be higher, and electroencephalogram (EEG) waves indicate a brain in the waking state.
Alertness is maintained by neurological circuits in the prefrontal cortex at the very front of the brain. Fibers from the prefrontal cortex communicate with the reticular formation, a neural structure located in the brainstem, to keep the cortex awake and alert. A brain injury that results in disruption of these connections results in coma.
Sleep is important to consider while studying for the MCAT or any other major exam. While it may be tempting to pull all-nighters in an attempt to maximize your test score, this may not be the best strategy for success. In fact, long-term sleep deprivation has been linked with diminished cognitive performance as well as the development of chronic diseases such as diabetes and obesity.
One of the best ways to enhance your recall and test performance is to maintain a regular schedule of sleep. Regular sleep, exercise, and a healthy diet help to make Test Day successful.
Sleep is studied by recording brain wave activity occurring during the course of a night’s sleep. This is done with electroencephalography, or EEG, which records an average of the electrical patterns within different portions of the brain. There are four characteristic EEG patterns correlated with different stages of waking and sleeping: beta, alpha, theta, and delta waves. There is a fifth wave that corresponds to REM sleep, which is the time during the night when we have most of our dreams. These sleep stages form a complete cycle lasting about 90 minutes.
An electroencephalogram (EEG) is a test used to monitor electrical activity in the brain. It consists of 19 recording electrodes placed on the scalp for 20 to 40 minutes, or continuously in the inpatient setting. This technique is used to study sleep and to identify areas of unusual brain activity, as seen during seizures.
Beta and alpha waves characterize brain wave activity when we are awake and are shown in Figure 4.6. Beta waves have a high frequency and occur when the person is alert or attending to a mental task that requires concentration. Beta waves occur when neurons are randomly firing. Alpha waves occur when we are awake but relaxing with our eyes closed, and are somewhat slower than beta waves. Alpha waves are also more synchronized than beta waves.
As soon as you doze off, you enter Stage 1, which is detected on the EEG by the appearance of theta waves, shown in Figure 4.7. At this point, EEG activity is characterized by irregular waveforms with slower frequencies and higher voltages.
As you fall more deeply asleep, you enter Stage 2. The EEG shows theta waves along with sleep spindles and K complexes, shown in Figure 4.8.
As you fall even more deeply asleep, you enter Stages 3 and 4, also known as slow-wave sleep (SWS). EEG activity grows progressively slower until only a few sleep waves per second are seen. These low-frequency, high-voltage sleep waves are called delta waves, shown in Figure 4.9. During these stages, it becomes especially difficult to rouse someone from sleep. SWS has been associated with cognitive recovery and memory consolidation, as well as increased growth hormone release.
Stages 1 through 4 are all part of non-rapid eye movement (NREM) sleep; rapid eye movement (REM) sleep is interspersed between cycles of the NREM sleep stages. In this stage, arousal levels reach that of wakefulness, but the muscles are paralyzed. It is also called paradoxical sleep because one’s heart rate, breathing patterns, and EEG mimic wakefulness, but the individual is still asleep. This is the stage in which dreaming is most likely to occur and is also associated with memory consolidation. Recent studies have associated REM more with procedural memory consolidation and SWS with declarative memory consolidation.
Remember the sequential order of these brain waves—beta, alpha, theta, delta—by combining their first letters to form BAT-D and remember that a bat sleeps during the day.
A sleep cycle refers to a single complete progression through the sleep stages. The makeup of a sleep cycle changes during the course of the night, as shown in Figure 4.10. Early in the night, SWS predominates as the brain falls into deep sleep and then into more wakeful states. Later in the night, REM sleep predominates.
Over the lifespan, the length of the sleep cycle increases from approximately 50 minutes in children to 90 minutes in adults. Children also spend more time in SWS than adults. Changes to sleep cycles from disrupted sleep or disordered work schedules can cause many health problems. Disruption of SWS and REM can result in diminished memory. Sleep deprivation also causes diminished cognitive performance, although the person who is sleep-deprived is unlikely to recognize that performance has been subpar. Sleep deprivation also negatively affects mood, problem solving, and motor skills.
Our daily cycle of waking and sleeping is regulated by internally generated rhythms or circadian rhythms. In humans and other animals, the circadian rhythm approximates a 24-hour cycle that is somewhat affected by external cues such as light. Biochemical signals underlie circadian rhythms. Sleepiness can partially be attributed to blood levels of melatonin, a serotonin-derived hormone from the pineal gland. The retina has direct connections to the hypothalamus, which controls the pineal gland; thus, decreasing light can cause the release of melatonin.
Cortisol, a steroid hormone produced in the adrenal cortex, is also related to the sleep–wake cycle. Its levels slowly increase during early morning because increasing light causes the release of corticotropin releasing factor (CRF) from the hypothalamus. CRF causes release of adrenocorticotropic hormone (ACTH) from the anterior pituitary, which stimulates cortisol release. Cortisol contributes to wakefulness.
The hypothalamic–pituitary–adrenal axis is an example of how the endocrine system can regulate behavior. The endocrine system is discussed in Chapter 5 of MCAT Biology Review.
Philosophers and those interested in the human experience have hypothesized about the purpose, meaning, and function of dreaming since antiquity. The ancient Egyptians believed that dreams were messages sent from the supernatural world to tell of future events. The Greeks believed dreams to carry messages from the gods, but the dream required the help of a priest to interpret. Dreams have long been a subject of wonder.
Most dreaming occurs during REM; however, soon after we enter Stage 2 sleep, our mental experience starts to shift to a dreamlike state. Throughout the night, approximately 75% of dreaming occurs during REM. REM dreams tend to be longer and more vivid than those experienced during NREM sleep.
While the purpose and meaning of dreams is not fully understood, a few theories have been proposed. In the activation–synthesis theory, dreams are caused by widespread, random activation of neural circuitry. This activation can mimic incoming sensory information, and may also consist of pieces of stored memories, current and previous desires, met and unmet needs, and other experiences. The cortex then tries to stitch this unrelated information together, resulting in a dream that is both bizarre and somewhat familiar. In the problem solving dream theory, dreams are a way to solve problems while you are sleeping. Dreams are untethered by the rules of the real world, and thus allow interpretation of obstacles differently than during waking hours. Finally, in the cognitive process dream theory, dreams are merely the sleeping counterpart of stream-of-consciousness. Just as you may be thinking about an upcoming weekend trip when your consciousness quickly shifts to your upcoming MCAT Test Day, so too does the content of a dream rapidly shift and change. Ultimately, the question is less Which group is right? and more How can we unify these theories? The study of dreaming is limited by the difference between the brain and the mind: dreaming must have a neurological component, but is still highly subjective. Neurocognitive models of dreaming seek to unify biological and psychological perspectives on dreaming by correlating the subjective, cognitive experience of dreaming with measurable physiological changes.
Other psychologists have proposed explanations for dreams. One of the most notable is Freud, who separated dreams into their manifest content (what one actually sees and hears) and their latent content (the underlying significance of these dream elements). Freud’s model of personality is discussed in Chapter 6 of MCAT Behavioral Sciences Review.
Sleep-wake disorders are divided into two categories: dyssomnias and parasomnias. Dyssomnias refer to disorders that make it difficult to fall asleep, stay asleep, or avoid sleep, and include insomnia, narcolepsy, and sleep apnea. Parasomnias are abnormal movements or behaviors during sleep, and include night terrors and sleepwalking. Most sleep-wake disorders occur during NREM sleep.
Insomnia is difficulty falling asleep or staying asleep. It is the most common sleep-wake disorder and may be related to anxiety, depression, medications, or disruption of sleep cycles and circadian rhythms. Narcolepsy, in contrast, is a condition characterized by lack of voluntary control over the onset of sleep. The symptoms of narcolepsy are unique, and include cataplexy, a loss of muscle control and sudden intrusion of REM sleep during waking hours, usually caused by an emotional trigger; sleep paralysis, a sensation of being unable to move despite being awake; and hypnagogic and hypnopompic hallucinations, which are hallucinations when going to sleep or awakening. Another dyssomnia is sleep apnea, which is an inability to breathe during sleep. People with this disorder awaken often during the night in order to breathe. Sleep apnea can be either obstructive or central. Obstructive sleep apnea occurs when a physical blockage in the pharynx or trachea prevents airflow; central sleep apnea occurs when the brain fails to send signals to the diaphragm to breathe.
Hypnagogic hallucinations occur when one is going to bed. Hypnopompic hallucinations occur when one is popping up out of bed.
Night terrors, which are most common in children, are periods of intense anxiety that occur during slow-wave sleep. Children will often thrash and scream during these terrors, and will show signs of sympathetic overdrive, with a high heart rate and rapid breathing. Because these usually occur during SWS, the child experiencing the episode is very difficult to wake, and usually does not remember the dream the next morning. Sleepwalking, or somnambulism, also usually occurs during SWS. Some sleepwalkers may eat, talk, have sexual intercourse, or even drive great distances while sleeping with absolutely no recollection of the event. Most return to their beds and awake in the morning, with no knowledge of their nighttime activities. Contrary to popular belief, awakening a sleepwalker will not harm the person; however, it is generally suggested to quietly guide the sleepwalker back to bed to avoid disturbing SWS.
Sleep deprivation can result from as little as one night without sleep, or from multiple nights with poor-quality, short-duration sleep. Sleep deprivation results in irritability, mood disturbances, decreased performance, and slowed reaction time. Extreme deprivation can cause psychosis. While one cannot make up for lost sleep, people who are permitted to sleep normally after sleep deprivation often exhibit REM rebound, an earlier onset and greater duration of REM sleep compared to normal.
Hypnosis, named after the Greek god of sleep, Hypnos, was first documented in the eighteenth century. Hypnosis can be defined as a state in which a person appears to be in control of his or her normal functions, but is in a highly suggestible state. In other words, a hypnotized person easily succumbs to the suggestions of others. Hypnosis starts with hypnotic induction, in which the hypnotist seeks to relax the subject and increase the subject’s level of concentration. Then, the hypnotist can suggest perceptions or actions to the hypnotized person. In practice, hypnosis is not the same as its sensationalized version in the media, in which a hypnotist will snap his fingers and cause an individual to exhibit bizarre behavior. Rather, hypnosis has been used successfully for pain control, psychological therapy, memory enhancement, weight loss, and smoking cessation. Brain imaging has indicated that hypnotic states are indeed real; however, effective hypnosis requires a willing personality and lack of skepticism on the part of the patient.
Hypnosis has been used to recover repressed memories of trauma; however, these memories are not admissible in a court of law. This is because the suggestible state of hypnotism makes an individual vulnerable to creating false memories, which can be perceived as completely real. False memories are discussed in Chapter 3 of MCAT Behavioral Sciences Review.
Defining meditation can be tricky and is highly dependent on the practitioner of meditation and his or her beliefs. Meditation has been a central practice in the religions of Buddhism, Hinduism, Taoism, Judaism, and others. Meditation usually involves quieting of the mind for some purpose, whether spiritual, religious, or related to stress reduction. In the secular Western tradition, meditation is often used for counseling and psychotherapy because it produces a sense of relaxation and relief from anxiety and worrying. To that end, meditation causes physiological changes such as decreased heart rate and blood pressure. On EEG, meditation resembles Stage 1 sleep with theta and slow alpha waves.
Recent studies have demonstrated that mindful meditation not only improves psychological well-being, but may even help improve test scores and student performance. Take time for yourself while studying for the MCAT; keep your mind calm to keep it sharp.