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Spontaneous movements in both stage 2 and REM steep are preceded by similar heart rate increase, beginning approximately 8 sec before onset the movement. It is suggested that the HR acceleration before spontaneous movement in sleep is not a gradual response to vascular congestion but, rather, may be triggered by internal arousal stimuli which, like movements, have sleep‐stage‐specific rules of occurrence.

Electrophysiological and self‐report data were obtained from 10 and 20 Ss, respectively, during 15 days of baseline, 30 days of 24‐hr per day exposure to a 660 msec, 3.5K Hz tone pulse with a 22 sec interstimulus interval (10 days each at 80, 85, and 90 dB), and during a 10‐day post‐exposure period. A self‐reported increase in difficulty falling asleep was not substantiated by objective sleep latency measures. Changes in total hours of sleep, number of awakenings, and percent time for sleep stages were of small magnitude and not consistently related to stimulus intensity. All 10 monitored Ss gave clear EEG and autonomic responses to the stimulus, with no evidence of response extinction over the 30‐day exposure period. There was no change in average all‐night heart rate. Total number of body movements during the night did not change. However, the movements that did occur, tended to be triggered by the stimulus, with most movements closely following the tone pulse. The youth and good health of the Ss, and the 24‐hr per day exposure, favoring rapid adaptation to the stimulus, are suggested to account for the lack of disruption of sleep.

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EEG and autonomic habituation were studied during sleep stages 2 and REM. and during awake. Repeated presentations of the same stimulus to the same subjects within sleep stages and when awake permitted evaluation of habituation between tone sets within stag 2, REM, and awake, and from sleep to awake. Forty‐six subjects were exposed to 800 Hz. 75 dB, 1‐ or 2‐sec tones, presented in sets of 20 tones. During steep, there was no habituation of the EEC response. Habituation occurred for both FPR and HRR during stage 2, with no evidence of within‐stage. between tone set habituation. No significant habituation occurred during REM for HRR or FPR. Evoked skin resistance responses were rare during both stage 2 and RKM. In the awake state, HRR was unresponsive but all other variables showed habituation. Tones presented during the night had no effect on the awake response.

Sleep loss is a ubiquitous phenomenon that occurs on many long-term field missions. The effects of sleep loss are, in general, detrimental to efficient functioning of man-machine systems. To illustrate the effect of sleep loss on task performance, data from four independent research institutes are reviewed. Data are presented relating to the prevention of sleep loss, and to the detection and minimization of sleep loss effects when they occur.