Quick Answer: The little brown bat sleeps approximately 19.9 hours per day, making it the record holder among well-studied species. Tigers, koalas, opossums, and pythons follow closely at 18-20 hours. Rankings shift depending on whether animals are measured in captivity or the wild, and on how researchers define sleep versus torpor.
In This Guide
Reading Time: 12 minutes
The question sounds simple. Which animal sleeps the most? But the moment you start pulling peer-reviewed sources, things get interesting fast. Published figures contradict each other. Studies measured in labs show different numbers than fieldwork. And the deeper question, what exactly counts as sleep, turns out to be contested territory in neuroscience.
This article focuses on the data itself: the rankings, the specific numbers, where those numbers come from, and why they sometimes disagree. If you want to know why these animals sleep so long, that is a separate evolutionary question we cover elsewhere. Here, we are focused on the evidence.
Top 10 Animal Sleepers: The Data Table
The table below draws from published sleep research. Where multiple studies exist, we show the accepted range. "Method" indicates how the data was collected: EEG (electroencephalography, the most accurate method) or BO (behavioural observation).
| Rank | Animal | Sleep per Day | Method | Context |
|---|---|---|---|---|
| 1 | Little Brown Bat | 19.9 h | EEG | Captive / laboratory |
| 2 | Tiger | 18-20 h | BO | Both captive and wild estimates |
| 3 | Koala | 18-22 h | EEG + BO | Captive high; wild lower end |
| 4 | Opossum | 18 h | EEG | Laboratory studies |
| 5 | Python | 18 h | BO | Post-feeding period especially high |
| 6 | Giant Armadillo | 18 h | EEG | Laboratory; wild data limited |
| 7 | Sloth | 15-20 h | BO + some EEG | Wide range; wild sloths sleep less than captive |
| 8 | Lion | 16-20 h | BO | Field observation; varies by feeding success |
| 9 | Hamster | 14 h | EEG | Widely used as sleep research model |
| 10 | Cat | 12-16 h | EEG + BO | Varies with age, environment, activity level |
A few things stand out in that table. First, bats are the only group with a clear EEG-confirmed number above 19 hours. Second, the tiger and lion rankings rely primarily on behavioural observation, which is the best practical tool for large wild carnivores but less precise than EEG. Third, the sloth's range (15-20 h) is wider than almost any other species because wild field studies, notably work published in the 2000s by Rattenborg and colleagues, showed far less sleep than captive EEG studies had suggested.
The Sloth Surprise: Captive vs. Wild EEG
Sloths were long assumed to sleep around 20 hours because captive EEG showed exactly that. When Rattenborg et al. attached EEG loggers to wild brown-throated sloths, the animals averaged only about 9.6 hours per day. The gap is striking and underscores how profoundly captivity changes sleep patterns across many species. Published totals for wild versus captive animals can differ by 50% or more.
8 min read
Captivity vs. Wild: Why Numbers Vary
This is the single most important factor in understanding why you will find conflicting figures online. A captive animal has no predators to watch for, no territory to patrol, and often abundant food. Remove those pressures and sleep duration climbs.
The effect has been documented across multiple orders. Lions observed in the Serengeti sleep about 19-20 hours after a large kill when threats are low. But during periods of active territorial defence, that figure drops considerably. Koalas in wildlife sanctuaries often hit the top of their published range (22 h) while wild koalas monitored with GPS and activity loggers cluster closer to 18.
Tigers present a particular challenge. There is almost no EEG data for wild tigers. The widely cited 18-20 hour figure comes from long-term behavioural observation at reserves and from captive zoo studies. It is plausible but should be understood as an approximation.
Dorothy, Sleep Specialist at Mattress Miracle: "When customers ask me about sleep duration, I always mention that context matters enormously. Even for humans, people who work shifts, have young children, or live with chronic stress sleep very differently from those in stable, low-stress conditions. Animals are no different."
For the hamster and opossum, the numbers are more robust. Both species have been used extensively in laboratory sleep research, meaning EEG data is plentiful and well-replicated. The opossum's 18 hours is one of the better-supported figures in comparative sleep science.
How Scientists Measure Animal Sleep
Understanding the data requires understanding how it is collected. There are two main approaches, and they produce genuinely different results.
Electroencephalography (EEG)
EEG is the gold standard. Electrodes placed on or in the skull record electrical activity across the cerebral cortex. During wakefulness, the brain produces fast, low-amplitude waves. Slow-wave sleep (SWS, also called NREM sleep) produces large, slow delta waves. REM sleep produces fast waves similar to wakefulness, combined with muscle atonia (paralysis of the major skeletal muscles).
EEG tells researchers not just whether an animal is sleeping, but which stage it is in. The challenge is that traditional EEG requires either a tethered cable or a surgically implanted wireless transmitter, both of which limit movement and require at least some time in a controlled setting. Modern miniaturised EEG loggers have expanded fieldwork possibilities, but wild EEG datasets remain far smaller than captive ones.
For the purposes of the ranking table above, EEG-derived figures are considered more accurate than behavioural estimates. When you see a precise number like 19.9 hours for the little brown bat, that comes from EEG-based lab work.
Behavioural Observation (BO)
Researchers watching animals in the field or in large enclosures record sleep based on behavioural cues: stillness, characteristic postures, closed eyes, reduced responsiveness to mild stimuli, and rapid reversibility (the animal wakes when touched or when a sound occurs). This method works at scale and in conditions where EEG is impractical.
The limitation is that behavioural sleep does not always match EEG sleep. Some animals show quiet wakefulness (eyes closed, still, but EEG shows alert brain waves). Others show brief microsleeps that BO misses entirely. In general, BO tends to overestimate total sleep time compared to EEG.
EEG in the Wild: The Miniaturisation Revolution
Rattenborg et al. (2016) published work showing that great frigatebirds engage in unihemispheric slow-wave sleep during multi-day transoceanic flights, with EEG logger data collected in flight. Similar miniaturised loggers have since been used with dolphins, fur seals, and sloths. This technology is slowly closing the gap between captive and wild sleep data.
What Counts as Sleep? Torpor, REM, and More
The biggest conceptual challenge in comparing animal sleep data is agreeing on what sleep actually is. This is not a trivial question.
True Sleep: The Two-Stage Definition
Most mammalian sleep researchers define sleep as a reversible behavioural state characterised by reduced responsiveness, specific posture, and, critically, alternating NREM and REM stages visible on EEG. By this definition, sleep is distinct from anaesthesia, coma, and torpor, even though all four involve reduced conscious awareness.
The little brown bat, opossum, giant armadillo, and hamster all show clear NREM-REM cycling, which is why their data is considered directly comparable to human sleep measurements.
Where Torpor Complicates Things
Torpor is a state of metabolic suppression seen in bats, some marsupials, hummingbirds, and other animals. Core body temperature drops significantly, heart rate slows, and metabolic rate falls to a fraction of normal. Animals in deep torpor are very difficult to rouse.
Here is where the debate enters. Brain-wave activity during shallow torpor sometimes resembles NREM sleep. During deep torpor, however, EEG becomes nearly flat, which does not match standard sleep definitions. Some researchers count only the arousal episodes within a torpor bout (when the animal briefly warms up and shows conventional sleep patterns) as true sleep. Others count the entire torpor period.
For bats, this matters enormously. Little brown bats spend much of winter in hibernation (extended torpor). The 19.9-hour figure comes from active-season measurements. Hibernation is a separate phenomenon.
Unihemispheric Sleep
Several species can sleep with one brain hemisphere at a time while the other remains alert. Dolphins, fur seals, many migratory birds, and possibly some other marine mammals do this. It raises a counting problem: does half a brain sleeping for 8 hours equal 8 hours of sleep, or 4?
Most researchers count unihemispheric slow-wave sleep (USWS) as sleep for the purpose of total daily sleep tallies, but note it separately because the function may differ from bilateral sleep.
What Animal Sleep Teaches Us About Our Own Needs
Humans are unusual among primates in having relatively short sleep durations (7-9 hours recommended by the Canadian Sleep Society) packed with high proportions of REM sleep. Understanding the extremes of the animal sleep spectrum, from the bat's 20 hours down to species that seem to need very little, helps researchers identify what the essential functions of sleep are and why those functions matter for health.
Honourable Mentions and Edge Cases
The top 10 table above focuses on well-documented species. But the animal kingdom has some fascinating edge cases worth noting.
The Giant Panda
Giant pandas sleep 10-16 hours per day. Their high sleep duration relative to similar-sized bears is thought to relate to the low caloric value of their bamboo diet, though the exact causal pathway is still studied. They do not make the top 10 but are frequently cited in popular articles.
Pythons After a Meal
Pythons are listed at 18 hours, but that number is not constant. Before feeding, a python may be quite active during nighttime hours. After consuming a large prey item (which can take weeks to digest), sleep duration spikes dramatically. This post-prandial sleep is thought to support the enormous metabolic demands of digestion in ectotherms.
Dolphins and the Sleep Paradox
Bottlenose dolphins have been observed resting one hemisphere at a time for years, apparently never losing consciousness completely. How much of this counts as "sleep" depends entirely on your definition. Studies using EEG in captive dolphins show that each hemisphere gets roughly 4-6 hours of slow-wave sleep per day, but the animal is never simultaneously fully asleep in the way a bat or koala is.
Insects: A Different Question Entirely
Insects, including honeybees and fruit flies, show states that look behaviorally like sleep and are regulated by similar neurochemicals (adenosine-related signalling in Drosophila). But insects lack a cortex, so there is no EEG analogue. Whether insect rest is "sleep" in the mammalian sense is an open and genuinely interesting research question.
Sleep Quality Matters as Much as Duration
At Mattress Miracle in Brantford, we think about this a lot. The animals that sleep 18-20 hours are often doing so on surfaces perfectly matched to their needs: bats hang in caves with ideal temperature and darkness, lions rest on warm ground in open savannah. The surface matters. If your mattress is working against your natural sleep stages, even 8 hours can leave you groggy. That is something our team has helped Brantford families address since 1997. If you are not waking refreshed, the surface underneath you is worth a conversation.
For anyone curious about the relationship between your sleep environment and temperature regulation, our guide covers how ambient conditions affect every stage of the sleep cycle. For Brantford families thinking about mattress options that support deep, restorative sleep, our showroom team can walk you through the science without the sales pressure.
If you find yourself fascinated by why certain animals evolved to sleep so much, take a look at our companion piece on the functional biology behind extreme animal sleep, covering the metabolic, evolutionary, and dietary reasons champion sleepers log such long hours. We also have a piece on the ecology and conservation angle of animal sleep research.
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Call 519-770-0001Frequently Asked Questions
Which animal sleeps the most per day?
The little brown bat holds the record at approximately 19.9 hours of sleep per day in controlled studies, followed closely by tigers and koalas. Rankings shift slightly depending on whether the animal is measured in captivity or the wild.
Do koalas really sleep 22 hours a day?
The 22-hour figure is frequently cited but represents the upper end of the range measured in captivity. Wild koalas average closer to 18-20 hours. Much of this rest involves quiet wakefulness, not full sleep, though EEG studies confirm extended periods of genuine slow-wave and REM sleep.
How do scientists measure how long animals sleep?
Two main methods are used. Electroencephalography (EEG) places electrodes to record brain-wave patterns, giving the most accurate data. Behavioural observation records posture, eye closure, and reduced responsiveness. EEG is considered the gold standard but is only practical in captive or laboratory animals.
Is torpor the same as sleep?
Not exactly. Torpor is a state of metabolic suppression that can look like sleep from the outside, but brain-wave patterns during torpor differ from those during conventional slow-wave or REM sleep. Some researchers count torpor episodes toward total rest time; others exclude them, which is one reason published figures vary.
Why does understanding animal sleep matter for humans?
Comparing sleep across species helps researchers identify which features of sleep are ancient and universal versus which evolved for specific ecological niches. This work informs our understanding of why humans need 7-9 hours and what happens when we consistently get less.
Sources
- Siegel, J.M. (2005). Clues to the functions of mammalian sleep. Nature, 437(7063), 1264-1271. doi.org/10.1038/nature04285
- Rattenborg, N.C., Voirin, B., Cruz, S.M., et al. (2016). Evidence that birds sleep during flight. Nature Communications, 7, 12468. doi.org/10.1038/ncomms12468
- Lesku, J.A., Roth, T.C., Amlaner, C.J., & Lima, S.L. (2006). A phylogenetic analysis of sleep architecture in mammals: the integration of anatomy, physiology, and ecology. The American Naturalist, 168(4), 441-453. doi.org/10.1086/506973
- Voirin, B., Scriba, M.F., Martinez-Gonzalez, D., Vyssotski, A.L., Wikelski, M., & Rattenborg, N.C. (2014). Ecology and neurophysiology of sleep in two wild sloth species. Sleep, 37(4), 753-761. doi.org/10.5665/sleep.3562
- Allison, T., & Cicchetti, D.V. (1976). Sleep in mammals: ecological and constitutional correlates. Science, 194(4266), 732-734. doi.org/10.1126/science.982039
- Campbell, S.S., & Tobler, I. (1984). Animal sleep: a review of sleep duration across phylogeny. Neuroscience & Biobehavioural Reviews, 8(3), 269-300. doi.org/10.1016/0149-7634(84)90054-x
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