Sleep

Sleep can be defined in many ways—behavioral, subjective, physiological—but the standard definitions of sleep and of its internal structure are derived from the patterns of electrical activity in the brain, which are recorded as an electroencephalogram (EEG) using surface electrodes on the head. EEG recordings during sleep reveal gradual, cyclic changes during the night in both the background frequencies and transient electrical events. These, in combination with recordings of muscular electrical activity, are used to define a number of standardized stages of sleep.

The five stages of sleep include the rapid eye movement (REM) stage and stages 1–4, which are the non-REM (NREM) stages. REM sleep dominates during the earliest stages of human development, but NREM increases during childhood, with the gradual emergence of the deepest NREM stages (3 and 4), characterized by the presence of slow EEG waves (delta waves). Newborn babies spend about 50 percent of their sleep time in REM sleep. REM sleep is typically stabilized by adolescence and accounts for 20 to 25 percent of sleep time. The precise age at which the deepest stage of NREM (stages 3 and 4) occurs has not been conclusively determined but evidence suggests a decline starting by age 20. Stages 3 and 4 are collectively called delta- or slow-wave sleep (SWS). In healthy, young adults, there is an orderly progression during sleep from the shallowest (stage 1) to deepest (stages 3 and 4) stages of NREM, followed by a period of REM sleep. The sleep pattern cycles back through stage 2 and then to REM regularly four to six times during the night, with a cycle length of about 90–120 min. NREM sleep, including SWS, dominates the first half of the night, while the second half includes more REM sleep and little or no SWS. In young adults, stage 2 occupies about 50 percent of the sleep period, and REM about 25 percent.

SWS is considered a deep-sleep stage because quite intense external stimuli are needed to arouse an individual from SWS, and reports of preceding mental activity after awakening from SWS are very limited. It is relatively easier to arouse people from REM sleep, and dream reports after arousal are vivid and often bizarre. NREM stages are characterized by reductions in physiological activity and more difficult arousal, while REM presents a picture of chaotic physiological activity and easier arousal. Typical REM features include rapid or unstable heart rate, respiration and temperature change; rapid horizontal eye movements; twitching of the extremities and facial muscles; and penile erection in males. The EEG pattern resembles that of an awake, aroused individual, despite continued sleep and a profound loss of muscle tone in the major postural muscles.