Sleep and wakefulness in rats: probabilities of state transitions and state-to-state serial correlations in relation to circadian and ultradian phases

Sleep and wakefulness were recorded from undisturbed male rats (n = 10) for 7 days in a 12:12 h light-dark cycle. Data were partitioned by circadian and ultradian phase before quantification of within-state transition probabilities and between-state serial correlations. Within each joint circadian-ultradian phase, short-term statistical patterns were identified, with quantitative variation between phases. Statistical "signatures" of sleep-wake pattern include positive serial correlations between durations of contiguous states, and between state bouts and inter-bout intervals. There was no correlation between wake bout duration and subsequent sleep episode duration, contradicting an oft-cited assumption of the sleep homeostasis hypothesis. The data confirm a positive correlation between the duration of rapid eye movement (REM) sleep and duration of the subsequent inter-REM interval, which is traditionally interpreted as evidence for a short-term REM homeostat. The present study concludes that the REM homeostat hypothesis is underdetermined. It is suggested that this serial correlation and other sleep-wake patterns can be explained by an alternative non-homeostatic mechanism; transition probability prevailing at the end of a state episode is carried over to the beginning of the next state ("inter-state transition momentum"). The plausibility of this hypothesis was supported by preliminary simulations using a 2nd-order 3-state semi-Markov model. HighlightsO_LIWake-related transition probabilities underly circadian and ultradian rhythms. C_LIO_LIStatistical signatures of short-term patterns of sleep-wake state were identified in all circadian and ultradian phases. C_LIO_LITransition probabilities varied with time in state. C_LIO_LIState transition probabilities were influenced by duration and identity of prior state. C_LIO_LISleep homeostasis hypotheses were not supported. C_LIO_LIA hypothesis of state-spanning transition momentum is proposed. C_LI