Matteo Cerri is a hibernation researcher at the University of Bologna, Italy. He is currently consulting for the European Space Agency about ways to make humans hibernate during long space missions.

Hibernating mammals are able to actively suppress their metabolism, meaning they can tell their body to use less energy. Hibernation is a marvelous physiological and molecular event, and it's still a mystery how the behavior is activated and regulated. One of the most curious mysteries about hibernation that I and my fellow hibernation researchers are trying to answer is why hibernating animals are so tired when they wake up.

There are several types of hibernation, which can last an entire season or just a part of a day (this is called "torpor"), and can even happen when the ambient temperature is high (which is called "aestivation").

For sure, the brain plays a key role in starting the entire chain of events, but how and which part is still unknown. Among the many unexpected facets of hibernation, one is incredibly surprising.

Traditionally, hibernation is commonly seen as a "big sleep," a way for animals to stave winter off when no food is around. But it's actually not. Hibernation is a state characterized by the active inhibition of metabolism, and in this state, the activity of the brain differs substantially from sleep and may in fact be closer to wakefulness than many people realize. Hibernators are known to wake up periodically from their "cold sleep," and most people would think "it's to eat, of course!"

But that is not the case. Hibernators don't eat during hibernation season (and, for what it's worth, they also don't drink or produce any urine). So, why are they waking up? To check out the weather?

Electroencephalographic recordings of the brain of hibernators give a surprising answer: They wake up to sleep. And it's not like they shift from hibernating to a nap. These animals wake up and pass out like they're exhausted. Delta brainwave readings, which can be used to measure the deepness or intensity of sleep, show that animals that have just woken up from hibernation are indeed sleeping intensely.

This observation has been confirmed both in seasonal hibernators, such as golden-mantled ground squirrels and European ground squirrels, and in animals that perform torpor, such as the Djungarian hamster. Why this is the case is the subject of great debate among hibernation researchers, and it matters because my team and others around the world are working on research that could lead to the possibility of human hibernation. We'd like to know as much about the process as possible.

There are two main hypotheses. The first one suggests that sleep is such a deep and necessary process for the brain—that it serves such a vital role that the brain itself has to command the body out of hibernation to recover the sleep it's lost during hibernation. In fact, the idea that hibernation is more similar to wakefulness than it is to sleep is the subject of a recent study conducted by me and some of my colleagues at the University of Bologna in Italy.

This hypothesis has been tested with an interesting experiment. If a scientist disturbed a hibernator of this "recovery sleep" for a few hours after it wakes up, then it stands to reason that after that the animal would make up for this time when it actually does fall asleep (it would sleep for the same total length of time as hibernating animals that weren't deprived of sleep immediately after they woke up). if that sleep was so important, it would be recovered at the end of the deprivation period. In other words, if the animal had a sleep debt, that debt would have to paid, sooner or later.

The second hypothesis takes a different view of the whole process. Brain activity is strongly affected by hibernation, and the brain itself goes through some intense changes during hibernation. For instance, during hibernation, there is a process of disconnection of neurons. Many synapses are in fact re­absorbed by the brain in what is very similar to a transitory state of Alzheimer's disease. This disconnection is quickly reversed after an animal wakes up, rewiring the brain in the same way it was before, which brings back all the information that was stored in the neurons.

Brain activity recordings in this kind of suspended animation state did indeed resemble activity during wakefulness

During the re­connection process, which happens in the first few hours after an animal comes out of hibernation, the brain is in a highly plastic state. Therefore, it's thought that the EEG activity that we see during these stages is not real "sleep," but just a nonspecific pattern of neuronal reactivation. If this is the case, in the experiment described before, we should not see any recovery sleep after the sleep deprivation, which would suggest there's not sleep debt in first place. In other words, hibernation wouldn't actually be making the animals tired, they would simply sleep to reform these neural connections.

The experiment I've suggested has actually been performed, more than once. But the results are conflicting. A team at the University of Zürich, Switzerland, found evidence of sleep debt in hibernating animals. They even went as far as testing different durations of sleep deprivation, and showed that, during torpor, sleep debt accumulates 2.75 times slower than during wakefulness.

A separate experiment by a team at Berkeley and Stanford reported that no rebound was observed after sleep deprivation, and so did teams from the University of Alaska and the University of Groningen in the Netherlands.

How can we explain the conflicting results? They looked at different animals. The Zurich group looked at the Djungarian hamster, while the Berkeley group looked at the ground squirrel. The main difference between the two species is that hamsters undergo daily torpor (hibernation that lasts less than 24 hours), while ground squirrels are seasonal hibernators.

So, the answer to our initial question is that we still don't know why animals fall asleep immediately after they wake up from hibernating. No more experiments on the topic have been conducted since the ones I described. But, my own work at the University of Bologna in Italy has supported the idea that brain activity during torpor or hibernation is more similar to wakefulness than it is to sleep.

In this experiment, a torpor-­like state was induced for the first time in a rat, a non-­hibernator (our goal is to eventually open the way to human hibernation in the not-so-near future). Brain activity recordings in this kind of suspended animation state did indeed resemble activity during wakefulness, but the activity became slower and slower as body temperature decreased, as if the frames of a movie were being projected slower and slower as the movie progressed.

No one knows what it's like to be in a state of hibernation, and we don't know if hibernators are still somewhat conscious. Perhaps if we can teach ourselves to hibernate, we'll learn the answer. In the meantime, I'm hoping that research on this topic will flourish again.