Abstract
Objectives: Sleep latency, sleep duration, and sleep efficiency are among the main characteristics of sleep quality in humans. In different sleep deprivation protocols, NREM and REM sleep latencies demonstrate a robust circadian pattern. It is unknown whether other mammals also show circadian modulation in sleep latency. We have previously found that repeated 2h sleep deprivations alternated with 2h rest, redistribute sleep over 24 h in the rat. Noteworthy, NREM sleep remains circadian modulation under this protocol, whereas REM sleep does not. The latter suggests that, in contrast to humans, the circadian control over REM sleep is weak in the rat. Here, we apply the same protocol to investigate circadian modulation of sleep latencies in the rat. Methods: EEG and EMG recordings were performed in freely moving rats (n=8) exposed to constant dark conditions. Starting at the onset of subjective day (CT 0), twelve 2h-periods of sleep deprivation (SD) were alternated with twelve 2h-periods of rest (48 h). At the onset of the 2h rest periods of the second day, NREM and REM sleep latency were determined applying definitions similar to those of humans: NREM sleep latency - the time between release from SD and the first consolidated NREM sleep episode lasting >=30 sec; REM sleep latency - the time between the first consolidated NREM sleep episode after release from the SD and a subsequent REM sleep episode lasting >=30 sec. Results: NREM sleep latency showed minor variation across the six periods of rest. The latency to enter REM sleep showed a gradual but significant shortening in the course of the subjective day (from more than 20 min to ~10 min), followed by an abrupt increase at the start of the subjective night (>20 min). In the middle of the subjective night REM sleep latency decreased again to values comparable to those found at the end of the subjective day. At the end of the subjective night REM sleep latency increased again to ~20 min. As a result, REM sleep latency showed a complex bimodal circadian pattern (p<0.05, Duncan after ANOVA). Conclusion: The biomodal modulation in REM sleep latency can be caused by the changes in monoamines level and brain temperature, and may be related to the nocturnality of the rat. The absence of a circadian modulation in NREM sleep latency, and the presence of a bimodal circadian pattern of REM sleep latency are further indications for key differences in circadian sleep regulation between humans and the rat.