The lengths of sleep and wake episodes during 2 consecutive days of recording were measured in five rats lacking circadian rhythms owing to lesions of the suprachiasmatic nuclei. Total sleep (TS) episode lengths and the amount of NREM sleep and paradoxical sleep (PS) within each episode were examined in relationship to the lengths of the immediately preceding and the immediately following wake episodes. As putative measures of sleep intensity, average and maximum delta wave (1-4 Hz) incidence and amplitude within NREM were also examined in relation to adjacent wake episode lengths. For sleep episodes longer than 50 min (78% of daily sleep), TS episode lengths and amount of NREM within these episodes showed significant positive correlations with both prior and subsequent wake episode lengths. PS durations within sleep episodes also showed significant positive correlations with subsequent wake episode lengths, but little correlation with prior wake episode lengths. The results suggest that in the absence of sleep-wake circadian rhythms, sleep time is subject to short-term homeostatic regulation. Amounts of PS within sleep episodes were highly correlated (r = 0.84) with amounts of NREM. NREM delta wave incidence and amplitude showed no significant relationships with the lengths of prior or subsequent wake episodes, suggesting that variations in sleep intensity may not play a prominent role in the short-term homeostatic regulation of ad lib sleep. Delta wave incidence and amplitude were also not correlated with the duration of NREM episodes, but incidence during wake was positively correlated with wake episode duration, suggesting that delta density during wake may be an electrophysiological indicator of the propensity to sleep.