Physiological Changes during Sleep

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

Brain Activity?

For centuries, scientists believed that sleep was a period of brain inactivity, yet studies over the last 60 years have shown us that the brain remains active during 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, when compared to non-REM sleep. In fact, the brain is more active during REM sleep than when we are awake. Brain activity patterns during REM sleep are far more variable and random and resemble the pattern 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 the body is controlled by various mechanisms such as sweating, shivering and changing blood flow to the skin, so that there is a minimal fluctuation in body temperature around a set level during wakefulness. Our bodies’ start to lose some amount of heat to the environment just before we go to sleep, which some scientists believe this actually plays a crucial role in inducing sleep. During sleep, our central set temperature drops by 1 to 2°F which is the reason we utilize considerably less energy to maintain our body temperature. It is believed that one of the primary functions of sleep is to conserve energy in this manner.

Respiratory Changes

Our breathing patterns also change during sleep. Breathing while we are awake is usually irregular, since it is affected by emotions, speech, posture, exercise and other factors. As we gradually move from wakefulness to different stages of non-REM sleep, our breathing rate decreases slightly and becomes highly regular. On the contrary, pattern during REM sleep becomes variable again, accompanied with an overall increase in rate of breathing.

Cardiovascular Activity

One of the main roles of sleep is to give the heart a much needed break from the constant demands that accompany life. As compared to wakefulness, during non-REM sleep there is an overall decrease in blood pressure and heart rate. During REM sleep, however, there is a pronounced variation in cardiovascular activity, which leads to an overall increase in heart rate and blood pressure. Moreover, changes in blood flow that results in erection in males and swelling of the clitoris in females is characteristic of REM sleep. The reason for these neural and physiological variations during REM sleep is currently not known, although it may be a by-product of REM-related changes in nervous system activity or related to dream content.

Increased Physiological Activity

During sleep, many physiological activities decrease during sleep. For example, kidney function and the subsequent urine production are reduced. However, certain physiological activities can be maintained and even increased during sleep. For example, one of the main changes inflicted by sleep is a marked increase in the release of growth hormone. Certain activities associated with cell repair, digestion and growth is often greatest during sleep, indicating that cell repair and growth may be an important function of sleep.