Study Objectives: To investigate the relative contributions of the homeostatic and circadian processes on sleep regulation under conditions of severe sleep restriction. Design: The 13-day laboratory based study consisted of 3 x 24-h baseline days (8 h sleep opportunity, 16 h wake) followed by 7 x 28-h forced desynchrony days (4.7 h sleep opportunity, 23.3 h wake). Setting: The study was conducted in a time isolation unit at the Centre for Sleep Research, University of Australia. Participants: Fourteen healthy, nonsmoking males, aged 21.8 SD) years participated in the study. Interventions N/AMeasurements: Sleep was measured using standard polysomnography. Core body temperature (CBT) was recorded and continuously using a rectal termistor. Each epoch of sleep was assigned a circadian phase based on the CBT data (6 x 60-degree bins) and an elapsed time into sleep episode (2 x 140-min intervals). Results: The percentage of SWS decreased with elapsed time into the sleep episode. However, no change in the percentage of REM sleep was observed with sleep progression. Whilst there was a circadian modulation of REM sleep, the amplitude of the circadian variation was smaller than expected. Sleep efficiency remained high throughout the sleep episode and across all circadian phases.Conclusions: Previous forced desynchrony studies have demonstrated a strong circadian influence on sleep, in the absence of sleep restriction. The current study suggests that in the presence of high homeostatic pressure, the circadian modulation of sleep, in particular sleep efficiency and to a lesser extent, REM sleep, are reduced.