Quiet Sleep - Sleep Mechanics: Sleep Disorders

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Quiet sleep

Sleep specialists have called non-REM or quiet sleep "an idling brain in a movable body." During this phase, thinking and most physiological activities slow down, but movement can still occur, and a person often shifts position while sinking into progressively deeper stages of sleep.

To an extent, the convention of describing people "descending" or "dropping" into sleep actually parallels changes in brain wave patterns at the onset of non-REM sleep. When you are awake, billions of brain cells receive and analyze sensory information, coordinate behavior, and maintain bodily functions by sending electrical impulses to one another. If you're fully awake, the EEG will record a messy, irregular scribble of activity. Once your eyes are closed and your nerve cells no longer receive visual input, brain waves settle into a steady and rhythmic pattern of about 10 cycles per second. This is the alpha-wave pattern, characteristic of calm, relaxed wakefulness. Unless something disturbs the process, you will soon proceed smoothly through the four stages of quiet sleep.

Four stages of quiet sleep

Stage 1. In making the transition from wakefulness into light sleep, you spend about five minutes in Stage 1 sleep. On the EEG, the predominant brain waves slow to 4–7 cycles per second, a pattern called theta waves. Body temperature begins to drop, muscles become relaxed, and eyes often move slowly from side to side. People in Stage 1 sleep lose awareness of their surroundings, but they are easily jarred awake. However, not everyone experiences Stage 1 sleep in the same way: If awakened, one person might recall being drowsy, while another might describe having been asleep.

Stage 2. This first stage of true sleep lasts 10–25 minutes. Your eyes are still, and your heart rate and breathing are slower than when awake. Your brain's electrical activity is irregular. Large, slow waves intermingle with brief bursts of activity called sleep spindles, when brain waves speed up for roughly half a second or longer. About every two minutes, EEG tracings show a pattern called a K-complex, which scientists think represents a sort of built-in vigilance system that keeps you poised to be awakened if necessary. K-complexes can also be provoked by certain sounds or other external or internal stimuli. Whisper someone's name during Stage 2 sleep, and a K-complex will appear on the EEG. You spend about half the night in Stage 2 sleep, which leaves you moderately refreshed.

Stages 3 and 4. Eventually, large slow brain waves called delta waves become a major feature on the EEG. Together, Stages 3 and 4 are known as deep sleep or slow-wave sleep. Stage 3 becomes Stage 4 when at least half of the brain waves are delta waves. During deep sleep, breathing becomes more regular. Blood pressure falls and pulse rate slows to about 20%–30% below the waking rate. The brain becomes less responsive to external stimuli, making it difficult to wake the sleeper.

Deep, slow-wave sleep seems to be a time for your body to renew and repair itself. Blood flow is directed less toward your brain, which cools measurably. At the beginning of this stage, the pituitary gland releases a pulse of growth hormone that stimulates tissue growth and muscle repair. Researchers have also detected increased blood levels of interleukin and other substances that activate your immune system, raising the possibility that deep sleep helps the body defend itself against infection.

Normally, young people spend about 20% of their sleep time in stretches of slow-wave sleep lasting up to half an hour, but slow-wave sleep is nearly absent in most people over age 65 (see "The later years"). Someone whose deep sleep is restricted will wake up feeling unrefreshed, no matter how long he or she has been in bed. When a sleep-deprived person gets some sleep, he or she will pass quickly through the lighter sleep stages into the deeper stages and spend a greater proportion of sleep time there, suggesting that slow-wave sleep fills an essential need.

Last updated:	January 23, 2007