The first objective measurements of man's ability to obtain adequate sleep during prolonged space flight were made during the three manned Skylab missions. EEG, EOG, and head-motion signals were acquired during sleep by use of an elastic recording cap containing sponge electrodes and an attached miniature preamplifier/accelerometer unit. A control-panel assembly, mounted in the sleep compartment, tested electrodes, preserved analog signals, and automatically analyzed data in real time (providing a telementered indication of sleep stage). One subject was studied during each manned mission and, while there was considerable variation among individuals, several characteristics were common to all three: stage 3 sleep increased during the flight and decreased in the postflight period; stage 4 was consistently decreased postflight, although this stage was variable during the flight; stage REM (rapid eye movement) was elevated, and REM latency decreased in the late postflight period (after day 3 postrecovery); and the number of awakenings during sleep either showed no change or decreased during the flight. In only the 28-d mission (Skylab 2) was there a significant decrease in total sleep time; in that case it was a result of voluntarily reduced rest time and was not due to difficulty in sleeping nor frequent awakening. The subject on the 84-d mission (Skylab 4) experienced more difficulty in the first half of the flight, showing a decreased total sleep time and increased sleep latency, but this resolved itself with time. Sleep latency presented no problem in the other flights. While many of the findings are statistically significant, in no case would they be expected to produce a noticeable decrement of performance capability. These findings suggest that men are able to obtain adequate sleep in regularly scheduled 8-h rest periods during extended space flights. It seems likely, based upon these results, that the problems encountered in earlier space flights did not arise from the zero-g environment per se but possibly were a result of more restricted living and working areas in the pre-Skylab spacecraft.