Circadian timed wakefulness at dawn opposes compensatory sleep responses after sleep deprivation in Octodon degus

The circadian timing system in mammals is thought to promote wakefulness and oppose sleep drive that accumulates across the activity phase in diurnal and nocturnal species. Whether the circadian system actively opposes compensatory sleep responses in mammals with episodes of alertness consolidated at dawn and dusk is unknown. In the present study, an interaction between circadian timed arousal at dawn and compensatory sleep responses after sleep deprivation (SD) was examined in Octodon degus, a hystricomorph rodent with crepuscular episodes of wakefulness. Recovery sleep was compared after 6 hours and 12 hours of SD ending at either CT 21 or 12, just before the dawn, and just after the dusk crepuscular episodes of consolidated wakefulness, respectively. Total sleep time and NREM sleep after SD increased proportionally to the amount of sleep loss; however, compensatory sleep responses after SD were attenuated at CT 23, a circadian time when a crepuscular event of wakefulness occurs in this species. EEG slow-wave activity (SWA) and body temperature levels in the first two hours after 6 and 12 hours of SD ending at CT 12 were similar. However, both were significantly higher than after 12 hours of SD ending at CT 21, suggesting factors other than the amount of prior wake duration can influence SWA levels. This study provides evidence that the circadian arousal system opposes compensatory sleep responses at dawn by actively promoting wakefulness in this species.