Circadian rhythms of sleep and wakefulness in mice: analysis using long-term automated recording of sleep

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Abstract
Circadian rhythms of wheel-running activity and polygraphically defined wakefulness, rapid-eye-movement (REM) sleep and non-REM (NREM) sleep were continuously observed in 10 mice (Mus musculus) under both alternating light-dark (LD 12:12) and continuous darkness (DD) conditions. Sleep-wake state was determined automatically using a computer-based method that allowed continuous recordings from 60-280 days in duration. The sleep-wake state percentages (of the circadian cycle) obtained were in substantial agreement with other estimates for this or similar strains and showed no significant difference between LD 12:12 (wake 54.3%, NREM sleep 38.1%, REM sleep 7.6%) and DD (wake 53.1%, NREM sleep 39.9%, REM sleep 7.0%) conditions. All 10 mice exhibited clear circadian rhythms in each of the 3 states and wheel-running activity under both lighting conditions for the entire duration of observation. Probability functions, computed using stationary sections of data from all 10 mice, showed distinct waveforms for all 3 states and wheel running. These waveforms were remarkably similar under entrained and free-running conditions. This documentation of sustained circadian rhythmicity in sleep-wake state throughout observations of unprecedented length contradicts the currently common assertion that circadian control of sleep state is weaker than that of activity.