SLEEP

the normal periodic suspension of consciousness during which the powers of the body are restored. Sleep is an easily reversible and spontaneous state of decreased responsiveness to external stimulation. It contrasts with wakefulness, in which there is an enhanced potential for sensitivity and an efficient responsiveness to external stimuli. Sleep is a regularly recurring suspension of consciousness that serves recuperative and adaptive functions. Sleep usually requires the presence of flaccid or relaxed skeletal muscles and the absence of the overt, goal-directed behaviour often seen in wakeful organisms. An electroencephalogram (EEG) recording the electrical activity of the human brain shows a distinctive pattern during sleep. Humans normally sleep at night, though many species of animals that are active during that time sleep during the day. The normal human sleep requirement varies from 6 to 9 hours each night, with 7.5 hours being the average. A person's age determines his length and pattern of sleep. A newborn may sleep as much as 16 hours a day in intermittent periods, a two-year-old from 9 to 12 hours. In the normal maturation process during infancy and early childhood, a pattern of many short periods of sleep shifts to one of a single, long, uninterrupted period. Elderly people may revert to sleep patterns of childhood, napping often during the day and sleeping only a few hours at night. a normal, easily reversible, recurrent, and spontaneous state of decreased and less efficient responsiveness to external stimulation. The state contrasts with that of wakefulness, in which there is an enhanced potential for sensitivity and an efficient responsiveness to external stimuli. The sleepwakefulness alternation is the most striking manifestation in higher vertebrates of the more general phenomenon of periodicity in the activity or responsivity of living tissue. There is no single, perfectly reliable criterion of sleep. Sleep is defined by the convergence of observations satisfying several different motor, sensory, and physiological criteria. Occasionally, one or more of these criteria may be absent during sleep or present during wakefulness, but even in such cases there usually is little difficulty in achieving agreement among observers in the discrimination between the two behavioral states. Additional reading The standard reference on the physiology of sleep is Nathaniel Kleitman, Sleep and Wakefulness, rev. and enl. ed. (1963). Also see Ernest Hartmann, The Functions of Sleep (1973), an excellent summary of psychological and biological research on REM and NREM sleep functions; Anthony Kales and Joyce D. Kales, Evaluation and Treatment of Insomnia (1984), the most comprehensive work on this sleep disorder; and Jerrold S. Maxmen, A Good Night's Sleep: A Step-By-Step Program for Overcoming Insomnia and Other Sleep Problems (1981), a popular book that covers a range of sleep problems. David Foulkes Rosalind D. Cartwright