In the early days of sleep research, scientists believed that all but the most essential bodily and cerebral functions would shut down during sleep. It wasn’t until the advent of the electroencephalograph (EEG) in the late 1920s that researchers began to realize that sleep involves a startling amount of brain activity. By monitoring brain waves, scientists discovered five cyclic stages of alternately low and high brain-wave function. It generally takes 90 to 110 minutes for the brain to complete one cycle.
The first four stages are called NREM (non-rapid eye movement) sleep; the fifth stage is REM (rapid eye movement). The NREM phases comprise approximately 75% of the sleep cycle (50% in infants) and are characterized by very restful sleep, in which body movements, blood pressure, breathing, and basal metabolic rates are reduced by as much as 30% from normal wakeful levels.<+>18<+> There may be some dreaming during NREM sleep though you’re unlikely to recall them. The majority of remembered dreaming occurs during the REM phase.
In everyone except for infants, narcoleptics, and people deprived of sleep for more than 200 hours, NREM stages precede REM. However, you may skip around from various light and deep stages or completely omit the deep sleep stages after the second or third cycle of the night. Additionally, people commonly experience a period of up to two hours of “quiet wakefulness” between four-hour periods of regular sleep cycles. During this interval, sleepers are neither fully awake nor fully asleep, but resting, reviewing their dreams, their thoughts turned off.
This is the shortest phase, in which slowing alpha waves prepare you for the sleep state. As you drift in and out of wakefulness, your breathing, heart rate, metabolic rate, and body temperature begin to drop. Muscles start to relax; you may experience a sensation of falling, followed by sudden muscle contractions called hypnic myoclonia. You may experience hypnagogic (the act of falling asleep) dreams. You are easily awakened by external stimuli, at which point you may only remember a few fragments of hypnagogic images and thoughts.
This is the longest sleep phase, marked by a light level of sleep. The brain slows down into theta waves, with intermittent surges of rapid brain waves (spindles) followed by large, slow bursts of delta waves. Breathing, heat rate, metabolic rate, and body temperature continue to decline. You may be awakened easily by sound and movement.
As the brain progressively slows down to large, slow delta waves, you enter a deeper stage of sleep. Muscles go limp and breathing is slow and even. The sleeper may begin to sweat. During this stage and Stage 4, the body begins to restore itself. In people under 30, the endocrine glands release human growth hormone (HGH), which promotes cell division and organ growth. Awakening a person in Stage 3 sleep is fairly difficult to do, and the sleeper may experience grogginess for a few seconds or minutes after awakening. After two or three sleep cycles, this stage may disappear and you may go directly from Stage 2 to REM sleep.
This is the deepest stage of sleep. Delta waves become larger and much slower than in Stage 3, and breathing, heart rate, metabolism, and temperature reach their lowest levels. Muscles continue to be inactive and the sleeper may sweat, but the body continues its restorative activities. Some sleepers may experience night terrors or sleep-walking. Toward the end of this stage, sleepers may readjust their position and may experience a muscular contraction as they enter the next stage. Rousing sleepers from this phase take a great deal of effort. Once awakened, people feel groggy and may require several minutes to orient themselves. After two or three cycles, this stage may disappear for the duration of the night.
This is the dreaming phase, marked by small, rapid alpha waves that resemble those indicating wakefulness and sensory awareness of external stimuli. However, in this stage our brains are reacting to internally generated stimuli from our dreams. All other muscles seize up to prevent us from reacting to the action in the dreams, but the sleeper may experience some slight twitching in the face, fingers, and toes. Men experience penile erections. Breathing and heart rate speed up and slow down in reaction to dream content. A sleeper in the REM stage is difficult to rouse and, if awakened, they will have a hard time adjusting to reality. This stage becomes progressively longer with each cycle, possibly lasting 60 minutes in the fifth cycle.
