ABSTRACTSleep onset following three kinds of evening activities was investigated. These activities were six hours of strenuous physical work, six hours of intensive studying and mental work, and six hours of relaxation, i.e., watching TV, listening to records and reading magazines.Ss were 15 young men, each of whom participated in one activity per night for three non‐consecutive nights.Results showed that studying delayed sleep onset by an average of about six minutes, butSs took similar times to fall asleep after exercise and after relaxation. At the moment of sleep onset, heart rate, respiratory rate, rectal temperature and peripheral pulse volume all were significantly elevated after exercise, when compared to studying and relaxation. Both of these results combined were interpreted as indicating that the level of physiological, systemic activity is relatively unimportant for sleep onset, while the level of mental stimulation seems to play a more important rol