Study objectives: Emerging from daily torpor, Djungarian hamsters (Phodopus sungorus) show an initial increase in electroencephalographic slow-wave activity (power density between 0.75 and 4.0 Hz) during sleep that gradually declines. This feature is typical for sleep following prolonged waking and supports the hypothesis that sleep pressure increases during daily torpor. After hamsters were subjected to sleep deprivation or partial non-rapid eye movement sleep deprivation immediately following torpor, slow-wave activity remained high and decreased only when sleep was allowed. An analysis of the dynamics of the process underlying the build-up of sleep pressure during episodes of waking and torpor may provide insights into the regulation of normal sleep and wakefulness. We have analyzed in more detail the timecourse of the process that is common for waking and daily torpor and that could account for the subsequent increase in slow-wave activity.

Design: Continuous 24-hour recordings of electroencephalography, electromyography, cortical temperature, and electroencephalographic spectral analysis were performed. Torpor data of 28 hamsters and sleep-deprivation data of diverse durations collected previously in 15 hamsters were analyzed.

Setting: N/A.

Patients or participants: N/A.

Interventions: Sleep deprivation.

Measurements and results: Slow-wave activity invariably increased as a function of the duration of both prior waking and torpor. However, the time constant of the build-up of slow-wave activity was approximately 2.75 times slower during torpor compared to sleep deprivation. Brain temperature recorded during the torpor bouts was 10 degrees to 12 degrees C below euthermic brain temperature. Therefore, the temperature coefficient of the time constant for the slow-wave-activity increase is between 2.3 and 2.8, a range typical for biochemical processes.

Conclusions: We conclude that the processes occurring during daily torpor in the Djungarian hamster are similar to those occurring during sleep deprivation, but the build-up of sleep pressure during torpor appears to be slowed down by the lower brain temperature.