Quick Answer: The little brown bat (Myotis lucifugus) is widely considered the most sleeping animal, logging approximately 20 hours of sleep per day. Other extreme sleepers include the giant armadillo (18+ hours), koala (18-22 hours), and opossum (18 hours). These animals sleep so much primarily because of low-calorie, hard-to-digest diets and high energy demands relative to body size, not because sleep is optional for them.
In This Guide
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When someone asks what the most sleeping animal is, they are usually expecting a single definitive answer. The reality is a bit more complicated, because sleep is defined differently across species, varies with season and life stage, and has been studied with varying methodology. But there are clear leaders.
The animals that sleep longest per day are not lazy; they are highly adapted to their ecological niches in ways that make extended sleep biologically necessary. Understanding why the most sleeping animal sleeps as much as it does turns out to reveal a great deal about why sleep matters for all mammals, including humans.
The Most Sleeping Animal: The Little Brown Bat
The little brown bat (Myotis lucifugus), common across North America including southern Ontario, consistently tops lists of the longest-sleeping mammals with approximately 19-20 hours of sleep per day. This extreme sleep requirement reflects several converging biological realities.
Why Bats Sleep So Much
High metabolic demands: Flying is one of the most energetically expensive forms of locomotion. A bat in flight burns calories at a rate that would require a proportionally enormous amount of food for a resting animal. By sleeping almost all day and foraging only in the active hours (typically 3-4 hours around dusk and dawn in summer), bats conserve energy that would otherwise go to maintaining a high metabolic rate during inactive periods.
Thermoregulation: Many bat species, including the little brown bat, allow their body temperature to drop significantly during rest periods (daily torpor). The transition into and out of torpor involves deep sleep states. Entering torpor reduces caloric expenditure dramatically, and the associated extended sleep is part of this physiological package.
Small size, high energy: Small mammals generally have higher metabolic rates per unit of body mass than large mammals. They burn through energy reserves more quickly and require more recovery time. This is one reason small mammals (bats, shrews, hedgehogs) tend to be among the longest sleepers, while large mammals (elephants, giraffes, horses) sleep much less.
Body Size and Sleep Duration in Mammals
Allison and Cicchetti's landmark 1976 study in Science examined sleep patterns across 39 mammalian species and found significant correlations between sleep duration and factors including body weight, metabolic rate, brain-to-body-weight ratio, and position in the food chain. Smaller mammals with higher metabolic rates consistently showed longer total sleep time. A subsequent analysis by Capellini et al. (2008) examining 127 mammalian species confirmed the general body-size-sleep-duration relationship while identifying ecological factors (predation risk, diet quality) as additional modulators.
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Other Extreme Sleepers
Giant Armadillo (Priodontes maximus): 18+ Hours
The giant armadillo, found in South America, sleeps approximately 16-18 hours per day according to field studies using activity monitors. They are largely nocturnal, spending most daylight hours in underground burrows. Their diet of termites and ants is high-volume but requires significant excavation effort, and extended sleep conserves energy between foraging bouts.
Koala (Phascolarctos cinereus): 18-22 Hours
The koala is perhaps the most famous of the extreme animal sleepers. Its extraordinary sleep requirement comes directly from its diet: eucalyptus leaves are low in nutrition, high in toxic compounds (tannins, phenols), and require significant metabolic effort to detoxify. The koala's liver is specialised to process these toxins, but the process is energetically costly. Sleeping for up to 22 hours per day minimises the energy the koala expends on everything else, leaving metabolic resources available for digestion and detoxification.
Opossum (Didelphis virginiana): 18-19 Hours
Virginia opossums (found across North America, occasionally reaching southern Ontario) sleep approximately 18-19 hours per day. They are opportunistic omnivores whose nervous system shows primitive features suggesting their sleep patterns have ancient origins. They are also the only marsupials in North America.
Python (Python regius and relatives): Up to 18-24 Hours After Feeding
Snakes, particularly large constrictors, enter extended sleep or dormancy after feeding that can last 18-24+ hours. This is an extreme version of the post-meal metabolic slowdown; digesting a prey animal that may be half the snake's body weight requires massive energy diversion away from other systems. Outside of feeding periods, snake sleep patterns are more variable and complex to study.
| Animal | Average Sleep (hours/day) | Primary Reason for Long Sleep |
|---|---|---|
| Little brown bat | ~20 | High flight energy cost, torpor, small body size |
| Giant armadillo | ~18 | Energy conservation, underground lifestyle |
| Koala | 18-22 | Eucalyptus detoxification, low-nutrition diet |
| Opossum | ~18 | Metabolic rate, omnivorous foraging pattern |
| Sloth (three-toed) | 14-20 (wild); up to 20 captive | Very low metabolic rate, leaf diet |
| Tiger | ~16 | High-intensity hunting, carnivore energy balance |
| Cat (domestic) | 12-16 | Crepuscular hunter, prey-dependent energy scheduling |
| Lion | 10-15 | Energy conservation between hunts |
| Dog (domestic) | 12-14 | Variable; polyphasic sleep pattern |
| Human | 7-9 (adult recommended) | Complex brain function, large cortex maintenance |
Why Do Some Animals Sleep So Much?
Sleep science has grappled with the question of why animals sleep at all for decades. The answer appears to be: because the consequences of not sleeping are severe, regardless of species. Research by Rechtschaffen and Bergmann (2002) demonstrated that total sleep deprivation in rats leads to death within weeks, with cascading organ failure. This finding reinforces that sleep is not a passive state but an active biological necessity.
For extreme sleepers specifically, the high sleep requirements serve several functions:
Energy Conservation
For small mammals with high surface-area-to-volume ratios, maintaining body temperature during inactive periods is metabolically expensive. Sleeping (particularly with torpor) reduces this cost dramatically. A little brown bat in torpor may reduce its metabolic rate by 98% compared to its active state, effectively putting its body on standby to conserve the calories it obtained during its brief nightly foraging period.
Detoxification and Cellular Repair
Sleep is associated with glymphatic system activity in mammals: the brain's waste-clearance mechanism, which operates primarily during slow-wave sleep and removes metabolic byproducts including amyloid proteins. For animals whose diets involve processing significant amounts of plant toxins (koalas, sloths), extended sleep may provide more time for hepatic and neurological detoxification processes.
Memory Consolidation and Brain Maintenance
Siegel's (2005) comprehensive review of mammalian sleep functions in Nature identified memory consolidation, synaptic maintenance, and neuroplasticity as core sleep functions across species. Animals with complex cognitive demands (predators, social animals) tend to show significant REM sleep, which is associated with memory processing and emotional regulation.
Sleep Across the Animal Kingdom
Not all creatures sleep in a way that resembles mammalian sleep:
Insects: Show circadian rest periods with reduced responsiveness but whether this constitutes "sleep" in the mammalian sense is debated. Fruit flies (Drosophila) show sleep-like states that respond to the same pharmaceuticals that affect human sleep, suggesting ancient evolutionary origins.
Dolphins and whales: Use unihemispheric sleep (one brain hemisphere at a time) to maintain surface breathing while resting. Half the brain sleeps while the other remains active enough to control swimming and breathing.
Migratory birds: Some migratory species can fly for days or weeks with only brief microsleep episodes. Alpine swifts have been recorded flying for up to 200 days consecutively, sleeping on the wing in very brief intervals.
Jellyfish: A 2017 study found that Cassiopeia jellyfish show pulsing cessation cycles consistent with sleep-like states, suggesting sleep may predate the evolution of a centralised brain by hundreds of millions of years.
Ontario's Champion Sleeper
The little brown bat is native to Ontario and widespread across the province, including in the Brantford-Hamilton area. Ontario bat populations have been severely affected by white-nose syndrome, a fungal disease that disrupts their hibernation, causing bats to wake too frequently during winter and burn through their fat reserves. This disruption of bat sleep is causing population crashes across North America, highlighting how dependent these animals are on their extended sleep patterns for survival.
What Humans Can Learn from Extreme Animal Sleepers
It would be easy to look at a bat sleeping 20 hours a day and conclude that we should all be sleeping more. The more nuanced lesson is this: every species sleeps exactly as much as its biology requires, no more and no less. The pressure of evolution has calibrated sleep duration to match metabolic need, brain complexity, diet, and ecological risk.
Humans sleep 7-9 hours as adults because our biology, our large, energy-hungry neocortex and complex social cognition, requires it. Chronic short sleep (under 7 hours) in humans is associated with increased risk of metabolic disease, immune dysfunction, and neurocognitive decline. We are not built like elephants (3-4 hours of sleep) or horses (3 hours). Our sleep need is as specific to us as the bat's 20 hours is to them.
Why We Cannot Simply Sleep Less
Mignot's (2008) review of sleep function in PLoS Biology describes sleep as "the price we pay for plasticity": the metabolic and neural processes that allow learning, memory formation, and synaptic pruning can only occur efficiently during sleep. The human brain, with its extraordinary capacity for learning and adaptation, requires proportionally more of this maintenance time than simpler nervous systems. Attempting to reduce sleep chronically is not simply a matter of willpower; it progressively degrades the cognitive functions that sleep is specifically there to restore.
The practical upshot: prioritising sleep is not indulgence. It is accurate biological maintenance. The bat is not lazy for sleeping 20 hours. The koala is not indolent for sleeping 22. They are doing exactly what their physiology requires. So are we, when we give ourselves 7-9 hours on a supportive, comfortable sleep surface.
If you are consistently getting less sleep than you need, the environment matters. A mattress that causes you to wake early, a bedroom that is too warm, a partner's movement that disrupts your cycles, these are correctable. Come into Mattress Miracle and let us help you remove at least one barrier to getting the sleep your biology is asking for. Browse our mattress collection or visit our guide to falling asleep for practical next steps.
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Call 519-770-0001Frequently Asked Questions
What is the single animal that sleeps the most?
The little brown bat (Myotis lucifugus) is most consistently cited as the highest-sleeping mammal, averaging approximately 19-20 hours of sleep and torpor per day. The koala and giant armadillo are close competitors at 18-22 and 18+ hours respectively. Precise comparisons are difficult because sleep definitions and measurement methodologies vary across studies and species.
Do sloths really sleep 20 hours a day?
Early reports of sloths sleeping 20 hours per day were based on captive animals; wild three-toed sloths have been found to sleep approximately 9-10 hours per day in their natural habitat according to field studies using implanted electroencephalographic sensors. Captive animals in low-stimulation environments sleep significantly more. So the extreme sloth sleep numbers are partially an artefact of captivity rather than a true reflection of wild behaviour.
Why do koalas sleep so much?
Koalas sleep 18-22 hours per day primarily because their eucalyptus leaf diet is extremely low in nutrition and high in toxic compounds. Detoxifying these compounds requires significant hepatic (liver) energy expenditure. Sleeping reduces the energy demand from movement and thermoregulation, leaving metabolic resources available for the demanding process of making eucalyptus nutritious and non-toxic.
What animal sleeps the least?
Giraffes and elephants are among the shortest-sleeping mammals, averaging approximately 2-4 hours per day. Migratory birds including the Alpine swift can go without consolidating sleep for weeks during migration. These animals compensate with high-quality concentrated sleep (giraffes spend most of their brief sleep in REM) and, in the case of some birds, unihemispheric sleep that allows partial brain rest while remaining mobile.
Do bats hibernate?
Yes, many bat species including the little brown bat hibernate through winter in caves, mines, or underground structures (called hibernacula). Hibernation is a prolonged torpor state rather than normal sleep, during which body temperature drops to near ambient, heart rate drops to a few beats per minute, and metabolic rate is minimal. White-nose syndrome disrupts this hibernation, causing bats to wake repeatedly and burn their fat reserves prematurely.
Sources
- Allison, T., & Cicchetti, D.V. (1976). Sleep in mammals: Ecological and constitutional correlates. Science, 194(4266), 732-734. doi.org/10.1126/science.982039
- Capellini, I., et al. (2008). Phylogenetic analysis of the ecology and evolution of mammalian sleep. Evolution, 62(7), 1764-1776. doi.org/10.1111/j.1558-5646.2008.00392.x
- Siegel, J.M. (2005). Clues to the functions of mammalian sleep. Nature, 437(7063), 1264-1271. doi.org/10.1038/nature04285
- Mignot, E. (2008). Why we sleep: The temporal organization of recovery. PLoS Biology, 6(4), e106. doi.org/10.1371/journal.pbio.0060106
- Rechtschaffen, A., & Bergmann, B.M. (2002). Sleep deprivation in the rat by the disk-over-water method. Behavioural Brain Research, 69(1-2), 55-63. doi.org/10.1016/0166-4328(95)00020-6
- Lesku, J.A., et al. (2006). A phylogenetic analysis of sleep architecture in mammals: The integration of anatomy, physiology, and ecology. American Naturalist, 168(4), 441-453. doi.org/10.1086/507046
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