Introduction There were several reasons for investigating the effects of sleep deprivation in man. One was simply a curiosity about the unusual consequences of prolonged insomnia described by earlier investigators. Kleitman1and Tyler2cited in detail the hallucinations and "psychotic-like" symptoms that occur under these circumstances and also described subjects who developed short-lived "schizophrenic" psychoses. Psychological alterations, such as inattention, apathy, illusions, and hallucinations, which commonly appear after 36 to 50 hours of wakefulness, have been noted to coincide with an increase in high-frequency, low-amplitude waves on the electroencephalogogram.3-5In contrast to such spontaneous psychological disturbances, sleep-deprived subjects have been able to perform normally on specific tests designed to measure work capacity, psychomotor performance, intellectual acumen, and personality structure.1,8Measurements of the basal metabolic rate, blood sugar, alkaline reserve, blood and urinary adrenal steroids, etc., have not been found to change significantly with sleep deprivation.1,5