A data-driven measure of REM sleep propensity for human and rodent sleep.

Introduction: Mammalian sleep is characterized by alternations between episodes of rapid-eye-movement sleep (REMS) and non-REM sleep (NREMS). The phenomenon of REMS pressure, namely a drive for REMS that builds up between REMS episodes, is thought to govern the timing of these ultradian NREMS-REMS cycles. Prior analyses of NREMS-REMS cycles in mice suggested that time in NREMS is a primary contributor to REMS pressure. We previously introduced a REMS propensity measure defined as the probability to enter REMS before the accumulation of an additional amount of NREMS. Analyzing mouse sleep data, we showed that REMS propensity at REMS onset was positively correlated with REMS bout duration and with the probability of the occurrence of a REMS bout followed by a short inter-REMS interval, called a sequential REMS cycle. Methods: Here, we extend the analysis of NREMS-REMS cycling to human and rat sleep behavior. We compare REMS propensity measures computed from sleep data recorded in humans, mice, and rats. As REMS in humans is influenced by the circadian rhythm, we also analyze circadian modulation of the expression of NREMS-REMS cycles across the human sleep episode. Results: We find that, as in mice, human and rat sleep contain both short sequential NREMS-REMS cycles and longer single NREMS-REMS cycles, with differences in the timescales of cycle durations. Although rodents exhibit polyphasic sleep in contrast with the consolidated sleep of humans, the calculated REMS propensity measures in all three species show similar profiles as functions of time spent in NREMS. Importantly, positive correlations of REMS propensity at REMS onset with REMS bout duration were present in both human and rat data as previously found in mouse data, suggesting that time spent in NREMS also influences REMS duration in these species. In the human data, we identified nuanced changes in the occurrence of single and sequential NREMS-REMS cycles suggesting that increased percent time spent in REMS as the sleep episode progresses is not solely due to increased REMS bout duration. Conclusion: Results suggest that similarities in the regulation of NREMS-REMS alternation exist, despite temporal differences, in nocturnal polyphasic rodent sleep and diurnal monophasic human sleep.