Bats sleep, and many species are among the most dedicated sleepers in the animal kingdom. The often-cited claim that some bats sleep around 20 hours a day turns out to be real but heavily dependent on conditions like ambient temperature, which can push actual sleep time down to around 11 hours in the same species. What makes bat rest genuinely unusual is not just how long they sleep but the way they do it: upside down, often in deep torpor that blurs the boundary between sleep and something closer to suspended animation, and in roosts chosen with surprising precision for their microclimate.
How Long Bats Actually Sleep
The big brown bat (Eptesicus fuscus) has become the poster species for extreme sleep. Lab recordings found that at a warm ambient temperature of 33°C, roughly 83 percent of the recording period was occupied by sleep, which works out to about 19.9 hours per day. That figure is the one that gets repeated in textbooks and wildlife fact sheets. But the same study found that when the temperature dropped to 26°C, total daily sleep fell to around 11 hours, less than many house cats manage.1PubMed Central. Re-examining extreme sleep duration in bats: implications for sleep phylogeny, ecology, and function
That range matters because bats in the wild rarely experience a perfectly stable 33°C environment all day. Cave temperatures fluctuate, outdoor roosts swing with the weather, and seasonal changes push bats through a wide thermal window. The 20-hour figure, while not fabricated, represents a ceiling under specific conditions rather than a typical daily reality.
What Bat Sleep Looks Like on a Brain Scan
Bats cycle through the same basic sleep stages that other mammals do. Their brains produce both non-rapid-eye-movement (NREM) sleep and rapid-eye-movement (REM) sleep, the phase associated with dreaming in humans. Temperature affects these stages unevenly. At comfortable warmth, bats spend meaningful time in REM, but as temperatures drop, REM sleep shrinks dramatically. In big brown bats kept at 19 to 21°C for several days, REM sleep collapsed from about 2.5 hours to just half an hour. Below 19°C, normal sleep rhythms stopped being detectable on brain recordings at all, and at 5°C the signal went essentially flat.1PubMed Central. Re-examining extreme sleep duration in bats: implications for sleep phylogeny, ecology, and function
That flat signal does not mean the bat is awake. It means the animal has slipped into a different metabolic state entirely, which is where the line between sleep and torpor gets interesting.
Torpor, Hibernation, and the Gray Zone
Torpor is not quite the same thing as sleep, although bats seamlessly move between the two. During torpor, a bat’s body temperature drops, its heart rate slows, and its metabolic rate plummets. In species that hibernate, the minimum metabolic rate during deep torpor bouts can fall to roughly 6 percent of their normal baseline. Species that use daily torpor instead of full hibernation tend to drop to about 35 percent of baseline, and their torpor bouts are much shorter.2PubMed Central. Daily torpor and hibernation in birds and mammals
The distinction matters because hibernating bats can stay in deep torpor for days or weeks at a stretch, punctuated by brief arousals when their body temperature shoots back up. Those arousals are metabolically expensive and seem to serve essential maintenance functions. Daily torpor users, by contrast, dip into and out of reduced metabolism within a single day.
Tropical bats add another wrinkle. You might assume that species living in warm climates have no need for torpor, but some free-tailed bats in the tropics use remarkably short bursts of it. On warm days, they alternate between micro-torpor bouts and normal resting metabolism within just a few minutes. On especially hot days, they extend torpor over the hottest hours and tolerate body temperatures as high as 42.9°C. This strategy lets them store heat passively from the environment and avoid losing water through evaporative cooling.3PubMed Central. Tropical bats counter heat by combining torpor with adaptive hyperthermia Heart-rate monitoring of another tropical free-tailed species, Molossus molossus, confirms that these bats enter torpor-like states even at body temperatures around 32°C, well above what most people picture when they think of hibernation.4Royal Society Open Science. Heart rate reveals torpor at high body temperatures in lowland tropical free-tailed bats
So while people sometimes ask whether bats “really” sleep or just enter torpor, the honest answer is both. They do both, often in the same resting period, and the mix depends on temperature, season, and species.
Why Bats Hang Upside Down to Sleep
Sleeping inverted is so associated with bats that it feels like it needs no explanation, but the mechanics behind it are worth understanding. Bats have evolved tendons in their feet and toes that lock into a gripping position under the weight of the animal’s own body. When a bat relaxes and hangs, gravity actually tightens the grip rather than loosening it. This passive locking mechanism means the bat expends essentially no muscular energy to stay attached to its perch while asleep or in torpor.5Journal of Theoretical Biology. Why bats hang upside down: a biomechanical hypothesis
Hanging also provides an escape advantage. A sleeping bat that is disturbed can simply let go and be airborne almost instantly, dropping into flight without needing a running start or a leap. Most bat species lack the leg strength to launch from the ground the way birds do, so having gravity do the initial work of getting airborne is a meaningful survival benefit.
Not every bat sleeps hanging freely from a ceiling, though. Many species tuck themselves into crevices, behind bark, or inside rolled leaves. Some tent-making bats in the tropics chew along the midrib of large leaves until the sides fold down, creating a sheltered pocket. These bats rest in a huddle underneath the leaf tent rather than dangling from a cave roof. The common thread is a protected overhead roost, not necessarily an upside-down posture.
When Bats Sleep and When They Wake
Most bats are strictly nocturnal, sleeping through daylight and emerging around dusk. The timing of that emergence is tightly controlled by internal clocks synchronized to the light-dark cycle. Research on the tropical tomb bat (Taphozous melanopogon) found that light is clearly the primary cue driving the circadian rhythm of flight activity. But the bats did not respond to a single fixed light level as a trigger. On shorter days they flew out when it was very dark, around 0.1 lux, while on longer days they emerged while the sun was still visible on the horizon at 50 lux or more. Researchers proposed that the bats undergo a seasonal shift in their sensitivity to light, which allows them to maintain remarkably consistent emergence times on the civil clock throughout the year.6PubMed. ‘Rigid’ internal timing in the circadian rhythm of flight activity in a tropical bat
Across broader geographic scales, bat emergence appears to be linked to sun altitude rather than clock time. A study examining bat species along a range of latitudes found that emergence occurs at a consistent position of the sun below the horizon, regardless of location. Because higher-latitude locations have longer twilights, the actual clock time relative to sunset shifts, but the sun angle stays the same.7bioRxiv. The geography of nocturnality – emergence patterns of bats on a latitudinal gradient This consistency suggests bats are responding to an absolute light threshold rather than to some relative measure of how long ago the sun set.
The moon also plays a role, though a subtler one. Several bat species delay their activity on nights with a full or waxing moon, a behavior sometimes called “lunar phobia.” Bright moonlight may increase predation risk from owls and other visual predators, so bats compensate by waiting longer or foraging less intensively on well-lit nights.8Ecological Processes. Artificial light at night (ALAN) pollution alters bat lunar chronobiology: insights from broad-scale long-term acoustic monitoring
Where Bats Choose to Rest
Roost selection is one of the most consequential decisions a bat makes, and the criteria are more nuanced than “find a cave.” Temperature and humidity inside a roost vary with distance from the entrance, the roost’s structural material, and external weather. In a study of tri-colored bats in caves and culverts, microclimate temperature was best predicted by external temperature and distance from the nearest entrance. Culverts were far more influenced by outside conditions than caves were, which makes sense given that caves have thick rock walls buffering the interior.9PubMed Central. External temperature and distance from nearest entrance influence microclimates of cave and culvert-roosting tri-colored bats (Perimyotis subflavus)
Within a single cave, different species often select very different zones. Research in Sarawak, Malaysian Borneo, found that Emballonura monticola chose the hottest spots in a cave while Hipposideros diadema occupied the coolest zones.10PubMed. Microclimate and Physiological Effects in the Roosts of Cave Dwelling Bats: Implications in Roost Selection and Conservation in Sarawak, Malaysian Borneo These preferences likely reflect each species’ metabolic strategy and body size rather than random preference.
Caves are not always the top choice. In Australia, eastern cave bats were found roosting in a corrugated-metal shed where daytime temperatures averaged 30°C and peaked at 40°C, far hotter and more volatile than the well-buffered caves nearby, which stayed between 20 and 25°C. Lactating females specifically preferred the shed, apparently because the high daytime warmth helped sustain the energy demands of producing milk, even though the nighttime temperature there plunged to around 18°C.11Australian Journal of Zoology. Bats under a hot tin roof: comparing the microclimate of eastern cave bat (Vespadelus troughtoni) roosts in a shed and cave overhangs The takeaway is that thermal stability is not always what bats want. Sometimes they actively seek variable or extreme conditions because the metabolic payoff is worth it.
The Benefits of Sleeping in a Crowd
Many bat species roost in dense clusters, and social roosting does more than provide safety in numbers. Huddling directly changes the physiology of rest. In Natterer’s bats during hibernation, bats that huddled with others experienced almost 30 percent less evaporative water loss than solitary individuals, even after accounting for differences in metabolic rate.12PubMed. Huddling reduces evaporative water loss in torpid Natterer’s bats, Myotis nattereri
Water conservation may sound like a minor benefit, but during hibernation it has cascading effects. Bats periodically arouse from torpor partly to drink and rehydrate, and every arousal burns through fat reserves at a rate far higher than torpor itself. If huddling reduces water loss enough to eliminate even one or two unnecessary arousals over a winter, the energy savings compound. For a hibernating bat running on a fixed fuel tank of stored body fat, those savings can mean the difference between making it to spring and not.
Light Pollution and Its Effects on Bat Rest
Artificial light at night is increasingly recognized as a disruptive force for nocturnal wildlife, and bats are particularly sensitive. Light pollution disrupts circadian rhythms and can alter when, where, and how much bats forage. A study of a North American bat community found that the spatial reach of artificial light’s effects on bats extends further than previously appreciated, displacing activity across a broader area than the visible glow alone would suggest.13Global Ecology and Conservation. Far-reaching displacement effects of artificial light at night in a North American bat community
The interaction between artificial light and moonlight adds another layer of complexity. As noted earlier, several bat species naturally delay activity on bright moonlit nights. Long-term acoustic monitoring found that light pollution altered this lunar pattern in four of the species studied, and that artificial light was associated with decreased bat activity overall, independent of the moon’s phase.8Ecological Processes. Artificial light at night (ALAN) pollution alters bat lunar chronobiology: insights from broad-scale long-term acoustic monitoring In practical terms, a bat roosting near a streetlight or illuminated building may end up with a compressed foraging window, less food intake, and more time spent in the roost under metabolic stress.
Not every species reacts the same way. Some opportunistic bats actually exploit streetlights and illuminated structures because insects congregate there. But light-averse species, which include many of the slower-flying, clutter-adapted bats, tend to avoid lit areas entirely. This means light pollution can reshuffle an entire bat community’s composition, favoring bold generalists and pushing out sensitive specialists.
White-Nose Syndrome and Disrupted Hibernation
One of the most devastating threats to bat rest in North America is white-nose syndrome (WNS), caused by the fungus Pseudogymnoascus destructans. The fungus invades the skin of hibernating bats and causes them to arouse from torpor far more frequently than normal.14PubMed Central. White-nose syndrome increases torpid metabolic rate and evaporative water loss in hibernating bats Each arousal costs a bat a significant fraction of its winter fat reserves. A healthy hibernating bat budgets a specific number of arousals over the winter; a bat with WNS blows through that budget weeks or months too early and starves before spring.
The fungus also increases metabolic rate and evaporative water loss during torpor itself, compounding the problem. Infected bats are not just waking up too often; they are losing energy and water faster even when they stay in torpor. There is an ironic twist to the arousal cycle, though. Research has shown that the temperature spikes associated with bat arousals actually inhibit the growth of the fungus.15PubMed Central. Temperature shifts associated with bat arousals during hibernation inhibit the growth of Pseudogymnoascus destructans The brief warming may be a partial defense mechanism, but it comes at an enormous metabolic cost. Since the disease reached North America in the mid-2000s, it has killed millions of bats and driven some regional populations of species like the little brown bat and the tri-colored bat toward collapse.
For bat conservation, understanding the physiology of rest is not just academic. Whether the goal is protecting hibernation caves from human disturbance, managing light pollution near roosting colonies, or developing treatments for WNS, the details of how, when, and where bats rest are at the center of the problem.
Bats That Break the Nocturnal Rule
While the vast majority of bats are nocturnal, a handful of species are partially or fully active during daylight. Some Pacific island flying foxes forage in daytime, likely because they face fewer aerial predators in their island ecosystems. A few small insectivorous species in the tropics also show crepuscular or dawn-activity patterns, emerging well before full darkness or continuing to forage into early morning light.
These exceptions are rare enough to prove the rule. Bat anatomy, from their large eyes tuned to low light to their echolocation systems built for navigating darkness, is overwhelmingly optimized for a nocturnal life. The few daytime-active species tend to live in environments where the usual selection pressures favoring nocturnality, especially predation by hawks and other raptors, are relaxed. Their rest patterns are correspondingly shifted: they may sleep during the hottest midday hours rather than through the entire day, more closely resembling the siesta pattern of some tropical birds than the all-day sleep of a cave bat.