Sleep
A condition of body and mind such as that which typically recurs for several hours every night, in which the nervous system is inactive,...
What is Sleep?
We all have at least a vague notion of what sleep is, but that doesn't mean that defining this mysterious part of our lives is simple. After all, detailed analysis of our own sleep isn't really an option, given that we rarely know that we're sleeping when we're asleep. And even if we observe the sleep of others, so much of what they experience—changes in the functions of their brains and bodies—is not easily seen from the outside.
Sleep scientists have explored these changes in depth, and their definition of sleep is tied to characteristic patterns of brain waves and other physiological functions. Features in this section describe and explain the significance of typical sleep patterns, as well as a variety of factors that disrupt these patterns.
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
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 in Sleep
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.
