Slow waves in the sleep electroencephalogram after daily torpor are homeostatically regulated

Animals emerging from hibernation or daily torpor show an initial increase in electroencephalogram slow-wave activity (SWA, power density between 0.75 and 4.0 Hz) in non-REM sleep, which subsequently declines. These typical features of sleep following prolonged waking led to the interpretation that the animals incur a sleep deprivation (SD) during torpor. This hypothesis has recently been questioned because the increase in SWA disappears in ground squirrels when sleep deprived immediately following hibernation. Here we show that in Djungarian hamsters subjected to SD immediately after daily torpor a predictable increase in SWA occurs during recovery. This supports the notion that the hamsters must sleep to dissipate the pressure for SWA incurred during torpor. The similarity between sleep after waking and torpor may provide a key for understanding sleep regulation.