The Djungarian Hamster Is Sleep Deprived during Daily Torpor

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Sleep, daily torpor and hibernation are thought to be homologous processes. However, non-rapid eye movement sleep (NREM) sleep after daily torpor in the Djungarian hamster, or during euthermic periods between hibernation bouts in ground squirrels, is accompanied by an initially elevated electroencephalogram (EEG) slow-wave activity (SWA, EEG power density between 0.75–4.0 Hz) and a subsequent gradual decline. This pattern is similar to that seen after sleep deprivation and led to the hypothesis that hibernating animals are sleep deprived during the torpid state. The hypothesis has recently been questioned because the increase in SWA disappears in ground squirrels when sleep deprived immediately following hibernation. To investigated whether this is also the case in the Djungarian hamster EEG, electromyogram (EMG) and brain temperature were recorded continuously in three experimental conditions in a short photoperiod. 1.) Arousal from torpor (T). 2.) Arousal from torpor followed by 1.5 h sleep deprivation (T+SD). 3.) Sleep deprivation for 1.5 h at the same time of day torpor ends (SD). In all three conditions a clear increase in SWA was found. T+SD resulted in the highest increase in the first 30 min interval of recovery and this increase was significantly larger than after SD alone. The data clearly demonstrate that the increase in SWA after daily torpor in the Djungarian hamster is sleep related, and dissipates only in the course of NREM sleep. This result supports the notion that the Djungarian hamster is sleep deprived during daily torpor.