SUMMARY Eight subjects were studied on an irregular sleep/wake schedule which was designed so that prior wake time and the time of day when 4‐h sleep periods were taken were both balanced. Rectal temperature and the sleep EEG were measured throughout the 8‐d protocol. A purification method was used to estimate the depression of rectal temperature (masking) caused by the first and second halves of each 4‐h sleep period. On average this was 0.187 ± 0.015°C during the first 2h and 0.262 ± 0.017°C during the second 2h. Masking increases beyond the first 2 h of sleep, but any effects due to the phase of the rectal temperature when sleep is taken are very weak. However, masking did increase (that is, the temperature was depressed more) when the amount of prior wake time was greater than 4 h. When the effects of sleep content variables were considered also, masking was still greater in the second half of sleep and tended to increase with the amount of slow‐wave sleep, but it decreased with increasing amounts of time awake during a sleep period (sleep latency or sleep discontinuity). Some implications of these results for the mechanism of sleep‐induced masking