The Mind and Body During Sleep

This focus article examines what happens in the brain and body during sleep by discussing the different stages of sleep and their functions; the brain systems that regulate sleep; the importance of daily rhythms of sleep and wakefulness; and, the changes that occur in sleep across the life span. This knowledge can foster a greater sense of control over sleep by making sleep more understandable and less mysterious.

The Stages of Sleep

During the night, we pass through five stages of sleep that, collectively, are termed REM (rapid eye movement) sleep and non-REM sleep. REM sleep is commonly called dream sleep while non-REM sleep consists of stages 1, 2, 3, and 4 sleep.

When we close our eyes to go to sleep at night, we first enter a relaxed state of wakefulness that is characterized by a brain wave (EEG) pattern called alpha. Good sleepers usually spend less than ten minutes in relaxed wakefulness before falling asleep, and some spend as little as a few seconds in this state. Falling asleep almost immediately at bedtime means one of two things: you are very sleep deprived; or, you are a very good sleeper.

After spending a few minutes in relaxed wakefulness, you drift into stage 1 sleep. This is not a "true" sleep stage because it is really a drowsy, transitional state between waking and sleep from which we are easily awakened. You've experienced stage 1 sleep if you've ever found yourself "drifting off" while lying on the couch or during a boring lecture or meeting. Stage 1 sleep is characterized by a brain wave pattern called theta and the simultaneous disappearance of the alpha rhythm. Our bodies relax deeply during stage 1: respiration slows, the muscles relax, and heart rate drops.

Most people who are awakened from this stage will report that they were "drifting off" but were not really asleep. This state is often described as drowsy, drifting, floating, and wandering. Mental activity during stage 1 sleep involves an abundance of mental imagery that sleep researchers refer to as "hypnagogic imagery". Here there is a letting go of our usual conscious thought processes and awareness of the external world. Relaxation techniques facilitate stage 1 sleep.

Most good sleepers only spend a few minutes in stage 1 sleep before descending to stage 2 sleep. This is considered the first true sleep stage. It is defined by specific changes in brain wave patterns called sleep spindles and K-complexes that probably represent intermittent attempts by the brain to preserve awareness before we lose conscious awareness of our surroundings. Here we become even more detached from conscious thought and the outside world as our brain and body become more deeply relaxed. Although stage 2 is deeper than stage 1, it is still a light stage of sleep for we are easily awakened from it. Insomniacs underestimate how much sleep they obtain, in part, because they are more likely to perceive stage 2 sleep as wakefulness.

We spend about thirty to forty-five minutes in stage 2 before entering the deeper stages of sleep called stage 3 and stage 4. We produce delta brain wave patterns during these two sleep stages, which are collectively called deep sleep. During deep sleep we reach the lowest levels of physiological activity during the 24 hour day. Deep sleep is also a period of dramatically reduced blood flow and energy use for the brain, which is probably crucial for restoring energy that is used during the daily demands of self-conscious awareness (e.g., thinking). It is very hard to wake up from deep sleep because the brain has turned off its awareness of the external world. We spend about forty-five minutes in deep sleep, revert to stage 2 for a few minutes, and then enter rapid eye movement (REM), or dream sleep.

Dream sleep is an intriguing stage of sleep. It is characterized by rapid eye movements (hence the name rapid eye movement sleep), intense visual imagery in the form of dreams, significant physiological activity, and heightened emotional activity in the brain (particularly emotions such as fear and anxiety). Because REM sleep is such an active period for the brain and body, we are more likely to awaken from this sleep stage than deep sleep. Many areas of the brain are highly active during dream sleep, particularly those involved in emotions and stress, as evidenced by increased blood flow and energy expenditure (increased use of oxygen and glucose). However, the area of the brain that is involved in complex cognitive functions and self awareness- the frontal lobes - become inactive during dream sleep. Dream sleep is often called paradoxical sleep because, except for muscle paralysis so that we can't act our dreams, our heart rate, blood pressure, and breathing rate are as high or higher than during waking, and our brain wave patterns resemble wakefulness.

The eye movements that occur during dreaming do, in fact, appear to coincide with the actions in our dreams. Most individuals who are awakened from dream sleep will report a dream, and even though many of us don't remember our dreams in the morning, everyone dreams. The content of a dream is often detailed, has a continuous quality, and is vivid, bizarre, and sometimes frightening. Despite the fact that dream content is often impossible or improbable, we experience dreams as if they are real because there is a complete suspension of self-reflection (we are not aware that we are dreaming) and logic (impossible experiences of time, places, people, and actions) during dreaming.

During the course of a night's sleep, we progress from Stage 1 to Stage 4 and then through REM sleep in about ninety minutes. Therefore, a six hour sleeper will move through four of these one and one-half hour cycles during the night while an eight hour sleeper will experience about five of these cycles. We spend about 5 percent of the night in Stage 1, 50 percent in Stage 2, 20 percent in deep sleep, and 25% in REM. Early in the night, deep sleep periods are longer (sometimes lasting up to one hour) whereas REM periods last only a few minutes. Later in the night, deep sleep periods grow shorter and the duration of REM periods increase so that, by the final REM period of the early morning, REM sleep may persist for an hour. As a result, we obtain most of our deep sleep during the first half of the night and most of our dream sleep during the last half of the night. And because sleep grows lighter in the second half of the night, awakening are more prone to occur in the second half of the night.

The Functions of Sleep

Despite the significant advances in sleep research over the past half century, no one theory about the function of sleep has been accepted by most sleep researchers, nor is there agreement about the answer to the question "What is sleep?" Sleep used to be defined as a state of quiescence and rest but we have just seen that the brain and body can be very active during sleep. Based on brain imaging studies, there is general agreement that sleep can be defined by a hallmark feature- loss of self-consciousness- and that sleep serves a general function of enforcing energy conservation and restoration.

In fact, it may be that energy conservation and restoration is particularly important to the self-conscious human brain because it uses a tremendous amount of energy to plan, solve problems, reflect on itself, respond to a very complex stimuli, and ponder complicated thoughts.

The various stages of sleep appear to serve particular functions. For example, deep sleep affords the brain and body a profound state of rest by allowing it to go "offline" to recover from the energy that we expend during normal conscious wakefulness. In this sense, deep sleep may serve a major restorative function that renews physical and mental energy. Blood flow decrease to the brain during deep sleep so that it can be directed to the muscles to help replenish physical energy. There is also good evidence that our immune system turns on during deep sleep to combat illness, which is why we need more sleep when we are sick.

Deep sleep seems to be the most important stage of sleep for, if we are deprived of sleep, the brain will recover or "make up" deep sleep first. Furthermore, loss of deep sleep produces the greatest impairments in daytime functioning compared to the loss of other stages of sleep.

Dream sleep is viewed by leading sleep researchers as the brain's system for processing emotions, discharging emotional stress, consolidating memories, and regularly suspending daily self-consciousness. Dr. Allan Hobson, a leading dream researcher at Harvard Medical School, has characterized dream sleep as being similar to psychopathological conditions such psychoses.

For example, dream sleep is, like psychoses, characterized by illogical thinking, lack of self-awareness, and hallucinations (that is, we believe something is real during a dream even though it is imaginary). Why the brain insures that we experience such non-rational states on a daily basis is unclear. Nevertheless, the fact that this occurs every night during sleep suggest that loss of self-awareness is in some way beneficial to the brain.

Dream sleep is also involved in processing and saving newly learned information into memory. Like deep sleep, dream sleep is very important since the brain will try to make up for dream sleep deprivation in the form increased dream sleep (called REM rebound). However, the brain attempts to recover all of the deep sleep it loses but only half of lost dream sleep. This suggests that deep sleep is more important than dream sleep.

Stage 2 sleep is probably a less potent form of deep sleep and is also involved with energy restoration. Stage 1 sleep is more akin to a deep relaxation state than a true sleep state; its main purpose is to prepare the mind and body for the entry into sleep.

Sleep Control Mechanisms in the Brain

The brain contains two complex systems that control sleep- a wake system and a sleep system. In simple terms, the wake system promotes alertness during the day while the sleep system promotes sleep at night. These two systems work together so that the wake system is dominant for about sixteen hours a day while the sleep system is dominant for the remaining eight hours of the day. The wake system is strongest in the morning, late afternoon, and early evening but weakens for a period of time in the mid-afternoon when we feel tired and are designed to take a nap.

The longer the wake system is "on" or active during the day (that is, the more waking time we accumulate during the day), the greater the drive or pressure for nocturnal sleep. Sound sleep is the result of a strong sleep drive and sufficient prior wakefulness during the day. However, one cause of disturbed sleep on a particular night is low sleep pressure caused by too much strength in the wake system and an insufficient period of prior wakefulness.

The other brain control mechanism that regulates sleep and wakefulness is the brain's internal 24 hour clock. This clock is regulated by a structure in the brain called the superchiasmatic nucleus (SCN). The SCN contains a kind of pacemaker that acts as a "clock for all seasons" by being tuned to daily and seasonal changes in light. In mammals, changes in light and darkness at various times of the year play an important role as a signal for breeding and other behaviors such as eating, sleeping, and putting on weight.

In humans, the primary role of the SCN appears to be to regulate daily rhythms of sleep and wakefulness and seasonal changes on mood. (Some people tend to become more depressed in the winter months as light diminishes, a phenomenon known as Seasonal Affective Disorder, or SAD).

The SCN acts on changes in light and darkness by affecting the release of a naturally occurring hormone in the brain called melatonin. When sunlight enters the eyes, melatonin concentrations decrease, which promotes wakefulness and increased alertness by signaling body temperature to rise. Body temperature generally rises during the day except for a brief dip in the afternoon, and then hits its daily peak around 6:00 p.m.

We are typically most alert in the late morning and early evening when body temperature is highest. At nighttime, light ceases to enter the eyes, which causes the SCN to trigger the secretion of melatonin. Body temperature drops, alertness wanes, and the descent toward sleep begins. Body temperature continues to decline and reaches its daily low at around 4:00 am; it then prepares us for eventual wakefulness by beginning to rise before sunrise. Once we awaken, the cycle of increasing body temperature during the day and falling temperature at night repeats itself.

Changes in Sleep With Aging

Sleep patterns change from the beginning to the end of life, particularly regarding how much we sleep, the timing of our sleep, and the stages of sleep.

Adolescents sleep about nine hours per nights; young adults about eight, middle aged adults about 7.5, and the elderly slightly less. Thus, our sleep need seems to decline by about one hour during our adult years. However, the elderly tend to compensate for reduced nighttime sleep by increased daytime napping.

There is also a change in the quality of sleep as we age. By our 30's and 40's, less deep sleep is experienced. As a result, we sleep more lightly and experience more awakenings during the night. In fact, deep sleep almost disappears entirely by age seventy.

Although we are not sure why sleep quality and the need for sleep declines with age, it is likely due to the fact that the sleep drive (e.g., the sleep system) weakens with age like many other functions.