ABSTRACTThe diurnal sleep patterns of female nurses working night duty were compared to their nocturnal sleep patterns while they were working regular hours during the day. Continuous EEG, EOG, and EMG recordings were made at the end of 2 month periods of night and day duty respectively. Day and night sleep differed with respect to both duration and pattern. Despite an earlier onset, the major sleep period was shorter during the day than the night and seemed to be more interrupted later in the session. This finding is in keeping with the increased amount of Stage 1 and decreased amount of slow wave sleep during the day than the night. Although no differences were evident with respect to overall percent REM, differences in the distribution of REM did occur. REM sleep occurred sooner during day than night sleep and there was more of it during the first part of day sleep. Thus night duty seemed to affect the pattern of sleep stage distribution as well as the absolute amount of, not only total sleep, but also some sleep stages, such as Stage SS. It is an open question how the naps of extended duration taken while on night duty influence the pattern of sleep during the day.