Ambient temperature during torpor affects NREM sleep EEG during arousal episodes in hibernating European ground squirrels

Ambient temperature (Ta) systematically affects the frequency of arousal episodes in mammalian hibernation. This variation might hypothetically be attributed to temperature effects on the rate of sleep debt increase in torpor. We studied this rate by recording sleep electroencephalogram (EEG) in arousal episodes induced after 4 days of torpor at different Ta. Spontaneous torpor bout duration (TBD) varied with Ta: TBD was maximal at 5.5°C (10.7 days), and was reduced at lower (-5°C: 5.0 days, 0°C: 9.3 days) and higher Ta (10°C: 8.7 days, 15°C: 5.0 days). Slow wave activity (SWA) during non-rapid-eye-movement (NREM) sleep, an indicator for sleep debt, showed initial high values after torpor at Ta ranging from -5°C to 10°C. When torpid at 15°C, SWA was not increased in the subsequent arousal episode. The data are thus inconsistent with a rate of sleep debt explanation for the temperature dependence of TBD.