In order to clarify the relationship between sleep-wake and core body temperature rhythms in the delayed sleep phase syndrome (DSPS), we conducted simultaneous monitoring of these rhythms in seven patients with DSPS and nine healthy control subjects for 6-10 days during their conventional sleep-wake schedules. The sleep onset and offset times were determined visually from sleep logs, and the temperature data were fitted to 24-hour cosinor curves by the least squares method. The sleep onset and offset times and temperature nadir were delayed significantly in patients with DSPS compared with the control subjects (p = 0.01, 0.003 and 0.02, respectively). We also found that sleep length and the temperature nadir to sleep offset interval were significantly longer in the DSPS than the control group (p = 0.03 and 0.02, respectively). The latter finding suggests that the inability of the patients with DSPS to normally phase-advance their circadian rhythm may be a consequence of masking of the advance portion of their phase-response curve by the last hours of their prolonged sleep episodes.