Observations on 6 [male] subjects, each of whom underwent one or more sleepless periods of approximately 60 hrs.'' duration, a total of 13 such periods, showed that during a period of prolonged wakefulness (60 hrs.) cutaneous sensitivity to painful stimulation increases markedly, while there is no change in sensitivity to touch. Individuals sleep 1143 hrs. after 60 hrs.'' sleeplessness, instead of the usual 8-9 hrs. The sleep following such a period of wakefulness is characterized by a decrease in frequency of movement, time spent in stirring, and the sum total of movements.