Discover
Hibernation is one of the most extreme and best-controlled physiological states in nature. Animals switch their metabolism down to a few percent of normal, cool their bodies, and wake up weeks later without the damage that the same conditions would cause in us. This page explains the basics in plain language.
Hibernation and torpor
Torpor is a controlled reduction of metabolic rate, usually with a drop in body temperature. It is not sleep, not a coma and not freezing: the animal actively lowers the set-point at which it regulates its temperature and keeps regulating it.
Daily torpor lasts hours, typically during the rest phase, and is used by many small mammals and birds such as hamsters, mice, bats and hummingbirds. Hibernation is a season-long series of multi-day torpor bouts, each lasting from a few days to several weeks, interrupted by short periodic arousals in which body temperature returns to normal for roughly half a day to a day.
Who hibernates?
Small mammals
Ground squirrels, marmots, dormice, hamsters, hedgehogs and many bats show deep multi-day torpor with large drops in body temperature.
Bears
Bears reduce their metabolism strongly while their body temperature drops only a few degrees. They can go months without eating, drinking, urinating or defecating, and lose little muscle or bone.
Marsupials, birds and primates
Torpor occurs in marsupials, in birds such as hummingbirds and the common poorwill, and in primates: fat-tailed dwarf lemurs in Madagascar hibernate for months.
Beyond the cold
Torpor is not only a winter strategy. Animals also use it to cope with drought, heat, food shortage or after fires and storms, in the tropics as well as in polar regions.
How does it work?
Entering torpor starts with an active suppression of metabolism, followed by a fall in body temperature. The brain, especially the hypothalamus, controls the state: in mice, specific populations of hypothalamic neurons can trigger a torpor-like state when they are activated. Fuel use switches mainly to fat. Heart rate can fall from hundreds of beats per minute to a handful, and breathing becomes slow and irregular.
Rewarming during an arousal is fast and largely driven by brown adipose tissue and shivering. Why hibernators arouse periodically, despite the high energy cost, is one of the open questions in the field; restoring sleep, immune function and cellular housekeeping are among the explanations studied.
Built-in protection
Cooling, low blood flow and rapid rewarming damage organs in humans. Hibernators go through these cycles many times per winter with little lasting injury. Research shows protective adaptations at many levels:
- Cells and mitochondria that tolerate cold and low oxygen, and limit oxidative damage during rewarming.
- Blood that resists clotting during torpor, with platelets that are stored and released again on arousal.
- Muscle and bone that are largely preserved despite months of inactivity.
- Brain and synapses that retract during torpor and rebuild on arousal.
- Immune function that is suppressed during torpor and restored during arousals.
Can humans hibernate?
Humans do not hibernate naturally, but researchers study whether parts of the hibernation toolkit can be used in medicine. Examples are better preservation of donor organs, protection against injury from interrupted blood flow during surgery, stroke or cardiac arrest, treatment of muscle wasting, and drugs that copy the cell protection of hibernators. Space agencies study whether a torpor-like state could make long missions safer and lighter.
Inducing a regulated hypometabolic state in large non-hibernating mammals, including humans, remains an open scientific challenge. It is an active research area, not a clinical treatment.
Common myths
“Hibernation is a long sleep”
No. Torpor is a different physiological state. Many hibernators actually sleep during their arousals.
“Hibernators are frozen”
No. Body fluids do not freeze; the animal keeps regulating its temperature at a lower level.
“It only happens in winter”
No. Torpor is used year-round and in warm climates too, whenever energy is scarce.
“Waking an animal is harmless”
No. Each arousal costs a large share of the winter energy budget. Disturbing hibernating bats or hedgehogs can be fatal.
Learn more
Watch & listen
Talks, lectures and podcasts by researchers from the IHS community.
Go to Watch & listen →Member publications
The latest research papers by IHS members, searchable by topic and year.
Browse publications →Seasonal Highlights
Our quarterly selection of new findings on torpor and hibernation.
Read the Highlights →Written by the International Hibernation Society for a general audience. Suggestions or corrections from members are welcome via info@hibsoc.com.