[Is the animal in hibernal sleep awake?]

It has been recognized for a long time that hibernation and slow wave sleep are homologous processes for energy conservation. Numerous EEG studies have demonstrated that during entrance into hibernation rapid eye movement sleep (REM) disappeared under cerebral temperature below 25 degrees C and that in deep hibernation, animals were preferentially in NREM sleep. Hibernation was thought to be an extension of NREM sleep. Nevertheless, other observations suggest that hibernation is not an homogeneous state. For example, in deep hibernation the activity of single thalamic units occurs with periods of activation and decline. High unit activity is associated with high electromyographic (EMG) activity, whereas low unit activity is associated with low EMG activity. To test the hypothesis that NREM sleep would have a restorative function, the EEG SWA activity (EEG delta power) was recorded during an arousal from hibernation and the following euthermic bout. Contrary to expectations, EEG SWA was maximal after an arousal and declined during the euthermic period. These findings suggest that a bout of hibernation is not NREM sleep, but would be the equivalent of a sleep debt.