An animal retreats to its den for winter and reappears in spring. It is easy to imagine how pleasant it might be to sleep for months. Yet hibernation research reveals things that do not fit the picture of one long nap, including animals warming back up and then entering sleep.
The key question is less how long the animal has been asleep than how much energy its body has been using. To get through a season when food is hard to find, hibernating animals make major changes to the way they regulate metabolism and body temperature. Comparing bears with small mammals shows that there is more than one way to do this.
How Can We Tell Sleep Apart from a Drop in Metabolism?
Closed eyes and a motionless body cannot tell us what is happening inside. Sleep researchers use measures such as brain activity to distinguish rapid eye movement (REM) sleep from non-REM sleep. Hibernation researchers, meanwhile, pay close attention to metabolic rate estimated from oxygen consumption, body temperature, and heart rate.
Torpor is a state in which an animal regulates its metabolism, body temperature, and other functions downward. Daily torpor can last a few hours, after which the animal resumes activities such as feeding. Hibernation can involve repeated bouts of torpor lasting days or weeks.[1]
Think of ordinary sleep and hibernation as two buildings displaying the same “closed” sign while running their systems in different ways. During hibernation, the pace of energy use changes dramatically. The body continues to regulate itself; it has not simply switched off.
Why Warm Up in the Middle of Hibernation?
After spending time in a low-metabolism state, many small hibernating animals temporarily bring their body temperature back into its normal range. These periods of rewarming punctuate the winter, so hibernation cannot be described as falling asleep in autumn and never waking until spring.[1][2]
That raises an intriguing question: if the aim is to save energy, why spend it on warming up?
Rewarming takes energy. Researchers are investigating what these periodic warm-ups accomplish, including how they relate to sleep, but no single explanation has been established for all species.[1][2] Calling them regular “reboots” may help us picture the pattern, but it does not tell us what those reboots are for. Hibernation is a complex process that balances energy savings with necessary physiological functions.
How Do Bears Get Through Winter Without Cooling Down Very Much?
Some small hibernators lower their body temperature dramatically during torpor. Do bears cool down in the same way?
A 2011 study of American black bears found that their metabolic rate during hibernation was suppressed to about 25% of their basal metabolic rate. Their body temperature was regulated within a range of 30–36°C, fluctuating in cycles lasting several days.[6] The important detail is that metabolism fell to roughly one-quarter of the baseline, rather than falling by 25%.
This combination shows why body temperature alone cannot tell us how much energy an animal is saving. Nor does simply cooling a body automatically put it into hibernation. The bears in this study sharply reduced their energy use while maintaining a relatively high body temperature.
These figures come from a particular species under particular study conditions. Rather than treating them as universal settings for bears or other hibernators, we can use them as a concrete example of the different ways hibernation works.
Can an Animal Need Sleep After Torpor?
The relationship between torpor and sleep is more complicated than their shared appearance of stillness suggests. A 2019 study of gray mouse lemurs examined brain activity during and after daily torpor. It compared six captive males under conditions with ambient temperatures of approximately 25°C or 10°C.[3]
At around 25°C, both non-REM and REM sleep occurred during torpor. Under the colder ambient conditions, however, when body temperature fell below approximately 21°C, ordinary non-REM and REM sleep were greatly reduced or absent. After torpor, researchers also observed an increase in delta waves, which are associated with the depth of non-REM sleep.[3]
These findings show that time spent at a low body temperature cannot simply be counted as ordinary sleep. The study concerned daily torpor in a particular primate, though, so it would be premature to conclude that every animal becomes more sleep-deprived the longer it hibernates. How sleep appears depends on the species, body temperature, and depth of torpor.[2][3]
“Why sleep again after resting for so long?” is a useful starting point for recognizing that rest can involve different physiological states.
Are Winter Sleepiness and Low Mood a Form of Hibernation?
When winter makes us sleepy or reluctant to go outside, it is tempting to say, “I wish I could hibernate.” As a way of expressing a feeling, the metaphor makes sense. It does not establish that seasonal difficulties in humans work through the same mechanisms as animal hibernation.
The US National Institute of Mental Health (NIMH) describes seasonal affective disorder (SAD) as a type of depression with a recurring seasonal pattern. In winter-pattern SAD, low mood may be accompanied by oversleeping, overeating with cravings for carbohydrates, weight gain, and social withdrawal.[5]
Feeling sleepier in winter is not, by itself, enough to determine whether someone has SAD. Researchers are studying the roles of the body clock, melatonin, and other processes associated with seasonal changes in daylight. That does not justify saying that the body has predicted a food shortage and begun to hibernate.[5]
If low mood or pronounced sleepiness persists and affects work, school, or everyday life, there is reason to speak with a healthcare professional rather than wait for spring. It can help to explain when the changes began, how sleep and appetite have shifted, and whether the same thing happened in that season in previous years. Treatments for SAD include light therapy, psychotherapy, and medication.[5]
Could Technology Enable Humans to Hibernate?
Artificially inducing the energy-saving states seen in animals could have applications in medicine or spaceflight. Researchers are exploring that possibility.
In a 2023 study, ultrasound stimulation of the preoptic area of the hypothalamus induced a torpor-like state in mice, lowering both body temperature and metabolism. A system that adjusted stimulation based on measured body temperature maintained the state for more than 24 hours. A reduction in body temperature was also reported in rats, which do not normally enter torpor.[4]
These experiments involved rodents under controlled research conditions. They did not produce a device that lets people sleep through winter. The finding is best understood as a step toward investigating which neural circuits regulate metabolism and what would be needed to manipulate them safely. That is where both its significance and its limits lie.
To Understand Hibernation, Consider the Body’s Internal Pace
From outside a den, a hibernating animal may look as though it is doing nothing. Inside its body, however, metabolism is slowing, temperature is being regulated, occasional rewarming may occur, and sleep states are changing.
Alongside “How many hours did it sleep?”, asking “How much energy did it use, and what physiological state was it in?” gives us a much richer picture of hibernation. The combination of temperature and metabolism in bears, brain activity after torpor, and the energy cost of rewarming all reveal careful adjustments behind an animal’s motionless appearance.
For our own winter difficulties, we can enjoy the animal metaphor while paying close attention to specific changes in our sleep and mood. Keeping those two perspectives distinct leaves room for both curiosity about living things and the care we may need.
References (6)
[1] Ruf & Geiser (2015), Daily torpor and hibernation in birds and mammals. A comparative review of daily torpor and hibernation. https://pmc.ncbi.nlm.nih.gov/articles/PMC4351926/
[2] Heller & Ruby (2004), Sleep and circadian rhythms in mammalian torpor. A review of the relationship between sleep and torpor. https://pubmed.ncbi.nlm.nih.gov/14977404/
[3] Royo et al. (2019), Daily Torpor and Sleep in a Non-human Primate, the Gray Mouse Lemur (Microcebus murinus). An original study measuring brain activity with EEG. https://pmc.ncbi.nlm.nih.gov/articles/PMC6768945/
[4] Yang et al. (2023), Induction of a torpor-like hypothermic and hypometabolic state in rodents by ultrasound. An original study in rodents. https://www.nature.com/articles/s42255-023-00804-z
[5] NIMH, Seasonal Affective Disorder. Official information on seasonal affective disorder. https://www.nimh.nih.gov/health/publications/seasonal-affective-disorder
[6] Tøien et al. (2011), Hibernation in black bears: independence of metabolic suppression from body temperature. An original study of American black bears. https://pubmed.ncbi.nlm.nih.gov/21330544/
