The time courses of slow wave sleep (SWS) and EEG slow wave activity (SWA) were examined in relation to core body temperature (CBT) during extended sleep periods of 15 h. Ten subjects (4 male, 6 female; aged 18-29 years) slept in the laboratory for 3 consecutive nights (2 of 8 h, and the last of 15 h). Bedtime was as close as possible to subjects' habitual bedtimes, but was always between 23.00 h and 1.00 h, and was the same on all nights. Standard polysomnographic measures as well as rectal temperature were collected continuously each night. Using 3 h blocks, SWS and SWA declined with time asleep, then showed a significant increase in the final 3 h. Using a more specific measure with 15 min means, 8 of the 10 subjects showed SWA returns late in sleep, that were not related to preceding amounts of waking after sleep onset (WASO), rapid eye movement sleep (REMS), or WASO+REMS. The timing and magnitude of the SWA return was significantly associated with the phase of CBT as indexed by delay from sleep onset to CBT minimum. The findings are consistent with a 12 h rhythm of SWS and SWA that is related to the phase of the CBT rhythm, with the minor pole of SWS/SWA occurring independently of WASO and/or REMS.