“Do hibernating animals sleep for months on end?” Picture a bear curled up in its den, and it is an understandable question. Yet bears both sleep and wake during hibernation, and in some small animals, sleep appears again after their bodies warm back up.
The key to untangling this is to separate two questions: “How much energy is the body using?” and “What sleep state is the brain in?” Hibernation cannot be explained simply as a long nap, but that does not mean sleep never occurs during it. Looking at metabolism, body temperature, and brain waves helps reveal what lies behind that same motionless appearance.
What Is Torpor, and How Does It Differ from Everyday Sleep?
Torpor is a state in which animals regulate their bodily functions to reduce energy use, for example by suppressing metabolism and maintaining a lower-than-usual body temperature. Daily torpor may last only a few hours before an animal becomes active again and searches for food. Hibernation, by contrast, can involve repeated bouts of torpor lasting days or weeks. In a comparison of birds and mammals, Ruf and Geiser distinguish the two by features such as duration and the extent of metabolic reduction.[1]
Comparing only “how many hours the animal did not move” makes ordinary sleep difficult to distinguish from torpor. Sleep researchers use brain waves and other signals to identify non-REM sleep, REM sleep, and wakefulness. Hibernation researchers also need to consider metabolic rate, estimated from oxygen consumption, and changes in body temperature.
For example, a small animal that greatly lowers its body temperature and a bear that keeps its body relatively warm may both curl up in much the same way, but different things are happening inside them. If we call hibernation a “low-power mode,” the important question is not whether the power is on, but how the rate of energy use changes. Life processes, including the regulation of breathing and body temperature, continue.
Why Do Animals Warm Up Again During Hibernation?
Depicting months of hibernation as one uninterrupted stretch of torpor misrepresents how many small mammals spend the winter. Their periods of low body temperature are interspersed with episodes in which body temperature returns to its normal range.[1][2]
Think of a smartphone battery, and this pattern seems a little odd. In the middle of saving power, the animal goes through an energy-intensive process. Rewarming costs energy. Researchers are investigating why animals nevertheless warm up periodically, examining its connections with sleep and a variety of other physiological functions.
However, a single explanation such as “they wake up to sleep” cannot account for everything. The roles of rewarming, and how necessary it is, remain under investigation across different species and conditions.[1][2] Hibernation is all the more intriguing because it is not a simple system in which saving more energy is always better.
Can Bears Save Energy Without Cooling Down Much?
In a 2011 study of American black bears, metabolic rate during hibernation fell to about 25% of basal metabolic rate, while body temperature was regulated within a range of 30–36°C over cycles lasting several days. “About 25%” means that energy consumption fell to roughly a quarter of the basal level.[6]
This example shows why the idea that lower body temperature translates directly and proportionally into greater energy savings is incomplete. The study found that metabolism remained low for a time even after normal body temperature returned. The researchers concluded that much of the metabolic suppression could not be explained by the fall in body temperature alone.[6]
In addition, a 2026 study drew on recordings of brain waves, eye movements, and muscle activity from 16 captive American black bears, using records from six of those bears for manual reference scoring to validate a method for automatically classifying sleep during hibernation and in summer. It showed that non-REM sleep, REM sleep, and wakefulness can be distinguished during hibernation too.[7]
Treating “hibernation or sleep” as an either-or question makes these bears difficult to explain. Inside an energy-saving body, the brain moves between sleep and wakefulness. The numbers and sleep characteristics described here apply to the species and study conditions examined. They are not universal settings that can simply be applied to small hibernating animals as well.
If Animals Sleep After Torpor, Are They Sleep-Deprived During Hibernation?
A 2019 study of gray mouse lemurs offers a concrete example for exploring this question. The researchers examined brain waves and other signals during and after daily torpor in six captive males, under conditions with an ambient temperature of about 25°C or 10°C.[3]
At an ambient temperature of about 25°C, both non-REM sleep and REM sleep were observed during torpor. Under the colder conditions, when body temperature dropped below about 21°C, the brain-wave features of ordinary sleep became much less apparent or disappeared. In the approximately 25°C condition, the researchers also observed an increase in delta waves after torpor; these waves are associated with the depth of non-REM sleep.[3]
What this tells us is that time spent motionless at a low body temperature cannot simply be added up as “time spent sleeping well.” At the same time, whether the deep sleep that follows torpor has the same underlying mechanism as ordinary sleep deprivation remains debated. Heller and Ruby’s review also discusses alternative interpretations.[2]
It would be a stretch to turn this small primate study into a claim that “all hibernating animals repay a sleep debt in spring.” Rather than how long an animal appears to rest, what matters is which species was studied, at what body temperature, and what brain waves were recorded.
Is Human Sleepiness in Winter the Same Energy-Saving Response?
On a dark morning when getting out from under the covers feels unappealing, saying “I wish I could hibernate” is only natural. But that feeling cannot be explained by the mechanisms of animal torpor. To understand seasonal difficulties in humans, we need medical information about humans.
The US National Institute of Mental Health (NIMH) describes seasonal affective disorder (SAD) as a type of depression with a recurring seasonal pattern. Winter-pattern SAD may involve oversleeping, overeating with carbohydrate cravings, weight gain, and social withdrawal. Researchers are also studying the relationships among changes in daylight, the body clock, and substances such as serotonin and melatonin.[5]
A single symptom such as “I feel sleepy in winter” is not enough to identify SAD or conclude that the body is preparing for a food shortage. If low mood or pronounced sleepiness persists and interferes with work, school, or everyday self-care, that is a reason to seek medical advice without waiting for spring.
To prepare for a consultation, it can help to note when the changes began, how sleep or appetite has changed, and whether the same thing happened at that time of year last year. For example, instead of “The cold just makes me feel generally unwell,” you could be more specific: “Since autumn, I’ve struggled to get up in the morning, I’ve been late more often, and I no longer enjoy things I used to like.” This is not a diagnostic checklist; it is a way to organize and communicate what is troubling you.
Treatments for SAD include light therapy, psychotherapy, and medication, and a healthcare professional can help you consider options suited to your symptoms and health.[5] An animal analogy may ease feelings of self-blame, but it should not become a reason to postpone care you need.
How Far Has Research on Artificial Hibernation Progressed?
If a low-metabolism state could be produced when needed, it might prove useful in medicine or on long-duration spaceflights. This possibility has prompted research into inducing torpor-like states in animals.
In a 2023 study, Yang and colleagues used ultrasound stimulation of the hypothalamic preoptic area to lower body temperature and metabolism in mice. By adjusting the stimulation in response to measured body temperature, they maintained the torpor-like state for more than 24 hours. A decrease in body temperature was also reported in rats, which do not normally enter torpor.[4]
These findings come from animal experiments investigating the neural mechanisms and methods for controlling metabolism. They do not demonstrate a finished device that lets humans safely sleep for months. Simply cooling a body is also not the same as the regulated torpor that animals enter, and this research should not be read as a method for people to reproduce on their own.
Moving Beyond “How Many Months Did It Sleep?”
When looking at a hibernating animal, three questions together can make it easier to understand what is happening: “How much has its energy use fallen?” “How has its body temperature changed over time?” and “Does its brain show signs of sleep or wakefulness?”
Bears greatly reduce their metabolism while keeping their bodies relatively warm; small animals may cycle between deep torpor and rewarming. Even within what we call hibernation, sleep can appear in different ways. An animal lying motionless in a den is not an animal whose internal processes have stopped.
“Low-power mode” is a useful starting point for imagining how animals make it through a season by using less energy. Look beyond the analogy and distinguish the body’s energy use from the brain’s sleep, and hibernation becomes far more interesting. As for the difficulties people experience in winter, rather than explaining them away with a metaphor, take a concrete look at what has changed in everyday life.
References (7)
[1] Ruf & Geiser (2015), Daily torpor and hibernation in birds and mammals. A comparative review of daily torpor and hibernation. https://onlinelibrary.wiley.com/doi/10.1111/brv.12137
[2] Heller & Ruby (2004), Sleep and Circadian Rhythms in Mammalian Torpor. A review of the relationship between sleep and torpor. https://www.annualreviews.org/content/journals/10.1146/annurev.physiol.66.032102.115313
[3] Royo et al. (2019), Daily Torpor and Sleep in a Non-human Primate, the Gray Mouse Lemur (Microcebus murinus). A small study of captive animals. https://www.frontiersin.org/journals/neuroanatomy/articles/10.3389/fnana.2019.00087/full
[4] Yang et al. (2023), Induction of a torpor-like hypothermic and hypometabolic state in rodents by ultrasound. An intervention study in rodents. https://www.nature.com/articles/s42255-023-00804-z
[5] NIMH, Seasonal Affective Disorder. Official medical information about 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. Original research on American black bears. https://pubmed.ncbi.nlm.nih.gov/21330544/
[7] Tøien et al. (2026), Automated sleep scoring in hibernating and non-hibernating American black bears. Original research validating a sleep-classification method. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0352640
