How come hibernation doesn’t have a restorative function?

While reading an article on HowStuffWorks about hibernation (a state of energy conservation characterised by a lower body temperature, slower breathing, lower  metabolism…), I was a bit puzzled by the following sentence “When an animal awakes from hibernation, it exhibits many signs of sleep deprivation and needs to sleep a lot over the next few days to recover”. Slow wave activity, which is a marker for homeostatic sleep pressure, shows a marked increase after arousal from the animal from hibernation, thus supporting this statement. But how come an animal has to recover after having been in such a long state of decreased metabolism. Although the page on HowStuffWorks provides a nice explanation of hibernation and its comparison to sleep, it didn’t provide an answer to this question. So I asked it on Quora, with Matteo Cerri providing me an answer for this question, albeit an inconclusive one.

The question you ask still does not have an answer.

It is absolutely correct that animals after torpor/hibernation sleep like they were sleep-deprived. In particular is the NREM sleep homeostasis that is in play here. No clear REM sleep rebound was observed after hibernation / torpor, but a clear increase in NREM sleep delta power was recorded. Delta power is a measure of the ‘intensity of NREM sleep’ (to be more precise, is the intensity of the EEG signal, therefore we are talking here about a cortical phenomenon) and correlates with the duration of the previous episode of wakefulness. So the more an animal stays awake, the higher the Delta amplitude will be during the following episode of sleep. 

But what really is Delta power? The actual theoretical frame has to do with synaptic strength rescaling (Tononi’s work). Briefly, during wakefulness, you use your cortical synapses,  some synapses get stronger, some weaker because information is processed in your cortex every day (even if you don’t want too). So during the NREM sleep, the delta waves re-scale the synaptic strength of large population of neurons so that the unuseful information (the noise) gets cancelled and only the valuable information is maintained.

The increase in Delta power after hibernation seems unexplainable in this view (considering the lower metabolic rate and the changes in the EEG during hibernation /torpor, it is highly unlikely that any information is processed in the cortex during this time) so somebody tried to understand if the post-hibernation sleep was real sleep.
How? Sleep depriving the animal after the hibernation bout. So, if this sleep was really homeostatic, it would have been recovered, if not, not.

Both Irene Tobler and Craig Heller had done work on this topic but with opposite conclusion:
Irene Tobler with Tom DeBoer did a study in Djungarian hamster, from which they concluded that there is a sleep rebound after depriving the animal of post-hibernation sleep. Craig Heller concluded the opposite. The two experiment were conducted with different methods, so it is not easy to compare them, but still the issue is unsolved.

To come back to the question, I can see two possibilities:

1) The post hibernation sleep is real sleep (homeostatically regulated and therefore recovered after deprivation). That means that: a) Tononi’s theory is challanged. b) Sleep pressure is independent of energy expenditure, so it should mirror some other basic function in the brain that is purely a function of time.

2) The post hibernation sleep is an unique type of sleep (it is not homeostatically regulated and it is not recovered after sleep deprivation). This means: a) the increase in Delta power serves some other function, maybe it is a way the cortex ‘reboot’ itself. b) Tononi’s theory is incomplete, but can still hold, depending on future findings.

If you want to know more about hibernation and torpor, definitely check out the website at The Open University covering “Animal at the extremes”.

References (open access):
[1] Palchykova, S., Deboer, T., & Tobler, I. (2002). Selective sleep deprivation after daily torpor in the Djungarian hamster. Journal Of Sleep Research, 11(4), 313–319.
[2] Larkin, J. E., & Heller, H. C. (1999). Sleep after arousal from hibernation is not homeostatically regulated. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 276(2), R522–R529.