What Is Sleep?

Every night, nearly every person undergoes a remarkable change: we leave waking consciousness and for hours traverse a landscape of dreams and deep sleep. When we wake, we typically remember little or nothing about the hours that have just passed. Except in rare instances, we never contemplate and appreciate that we are sleeping while we are asleep. Thus, although everyone sleeps, most people would be hard-pressed to precisely define sleep. All organisms exhibit daily patterns of rest and activity that resemble the daily sleep and wakefulness patterns seen in humans. From observing changes in behavior and responsiveness, scientists have noted the following characteristics that accompany and in many ways define sleep:

• Sleep is a period of reduced activity.
• Sleep is associated with a typical posture, such as lying down with eyes closed in humans.
• Sleep results in a decreased responsiveness to external stimuli.
• Sleep is a state that is relatively easy to reverse (this distinguishes sleep from other states of reduced consciousness, such as hibernation and coma).

From observations of behavioral changes that accompany sleep and simultaneous physiological changes, scientists now define sleep in humans based on brain wave activity patterns and other physiological changes as described below.

Physiological Changes During Sleep

Many physiological variables are controlled during wakefulness at levels that are optimal for the body’s functioning. Our temperature, blood pressure, and levels of oxygen, carbon dioxide, and glucose in the blood remain quite constant during wakefulness. During sleep, however, physiological demands are reduced and temperature and blood pressure drop. In general, many of our physiological functions such as brain wave activity, breathing, and heart rate are quite variable when we are awake or during REM sleep, but are extremely regular when we are in non-REM sleep.

Brain Activity

For centuries, physicians believed that sleep was a period of brain inactivity, yet research over the last 60 years has shown us that the brain remains active during sleep. There is a progressive decrease in the activation or "firing" rate of most neurons throughout the brain as sleep progresses from wakefulness to non-REM sleep. Also, the patterns of neuron firing change from a seemingly random and variable activity pattern during wakefulness, to a much more coordinated and synchronous pattern during non-REM sleep.

During REM sleep (the stage of sleep most associated with dreaming) there is an increase in the firing rate of most neurons throughout the brain, as compared to non-REM sleep. In fact, the brain in REM sleep can even be more active than when we are awake. Patterns of brain activity during REM sleep are more random and variable, similar to during wakefulness. This pattern of brain activity during REM sleep probably underlies the intense dreaming that occurs during this state.

Body Temperature

Through a process known as thermoregulation, the temperature of our body is controlled by mechanisms such as shivering, sweating, and changing blood flow to the skin, so that body temperature fluctuates minimally around a set level during wakefulness. Just before we fall asleep, our bodies begin to lose some heat to the environment, which some researchers believe actually helps to induce sleep. During sleep, our central set temperature is reduced by 1 to 2°F. As a result, we use less energy maintaining our body temperature. It has been hypothesized that one of the primary functions of sleep is to conserve energy in this way. 

Body temperature is still maintained, although at a slightly reduced level during non-REM sleep, but during REM sleep our body temperature falls to its lowest point. Curling up in bed under a blanket during the usual 10- to 30-minute periods of REM sleep ensures that we do not lose too much heat to the environment during this potentially dangerous time without thermoregulation.

Respiratory Changes 

Our breathing patterns also change during sleep. When we are awake, breathing is usually quite irregular, since it is affected by speech, emotions, exercise, posture, and other factors. As we progress from wakefulness through the stages of non-REM sleep, our breathing rate slightly decreases and becomes very regular. During REM sleep, the pattern becomes much more variable again, with an overall increase in breathing rate.

Cardiovascular Activity

One of the possible functions of sleep is to give the heart a chance to rest from the constant demands of waking life. As compared to wakefulness, during non-REM sleep there is an overall reduction in heart rate and blood pressure. During REM sleep, however, there is a more pronounced variation in cardiovascular activity, with overall increases in blood pressure and heart rate. Additionally, changes in blood flow that cause erections to occur in males or swelling of the clitoris in females is characteristic of REM sleep The underlying reason for these considerable neural and physiological variations in REM sleep is currently unknown, and may be a by-product of REM-related changes in nervous system activity or related to dream content. 

Increased Physiological Activity During Sleep

For the most part, many physiological activities are reduced during sleep. For example, kidney function slows and the production of urine is decreased. However, some physiological processes may be maintained or even increased during sleep. For example, one of the greatest changes induced by sleep is an increase in the release of growth hormone. Certain physiological activities associated with digestion, cell repair, and growth are often greatest during sleep, suggesting that cell repair and growth may be an important function of sleep. 

Dreams

One of the most notable but least understood characteristics of sleep is dreaming, during which our thoughts follow bizarre and seemingly illogical sequences, sometimes random and sometimes related to experiences gathered during wakefulness. Visually intense dreaming occurs primarily during REM sleep. However, not all dreams occur during REM sleep. For example, night terrors actually occur during non-REM sleep. 

Varying explanations for dreaming, as well as the meanings of dreams, have been offered by philosophers and psychologists throughout history. Even with recent scientific investigations of dreaming, our dreams still remain something of a mystery. Some experts suggest that dreams represent the replay of the day’s events as a critical mechanism in the formation of memories, while others claim that the content of dreams is simply the result of random activity in the brain.