Abstract
Purpose: Characterize the influence of the circadian clock on vigilance state episode frequency and duration. Methods: Rats (n=8) were implanted with EEG and EMG electrodes and subsequently adapted to constant dark conditions (DD) for at least 1 week. A baseline (BL) day was recorded followed by “short-day protocol” - 2h sleep deprivation followed by 2h rest for 2 days. Vigilance states were determined and EEG spectral analysis was performed. State episode frequency and duration was determined based on episode consolidation and interruptions. Data from the second day in the protocol is analyzed and compared with baseline. Results: During the protocol the animals slept 7.5% less over the 24-h circadian cycle. Vigilance states and slow-wave activity of the NREM sleep EEG (SWA 1-4 Hz) were evenly distributed across the day. REM sleep and SWA did not show a circadian modulation whereas the circadian modulation in NREM sleep and waking were markedly reduced. Waking and NREM sleep episode duration and episode frequency of all vigilance states showed a circadian modulation during the baseline. During the protocol episode frequencies of all vigilance states lost their circadian rhythm. NREM and REM sleep frequency increased to the highest values found in baseline, whereas waking frequency decreased to the lowest levels. REM sleep episode duration lost its circadian rhythm. The circadian modulation of waking episode duration remained intact except for CT20-24 where the duration was decreased. NREM sleep episode duration increased between CT16-18, reducing circadian amplitude. Conclusions: In the present protocol circadian modulation in sleep and waking is mainly caused by circadian modulation in vigilance state episode duration. Episode frequency was not under strong influence of the circadian clock. In contrast to humans, the influence of the circadian clock on REM sleep was weak compared to waking and NREM sleep.