Animals sleep in an enormous range of places, from underground burrows and treetop nests to the open ocean surface and even mid-air. Where an animal sleeps depends on its size, its predators, its body temperature needs, and whether it lives on land or in water. Some of these sleeping arrangements are surprisingly creative.
Burrows, Dens, and Caves
Underground or enclosed shelters are among the most common sleeping sites in the animal kingdom. Foxes and snakes sleep in burrows they dig or inherit from other animals. Bears retreat to caves or dens. Lions return to dens that might be caves, dense bushes, or spaces beneath large trees to rest between hunts. Crocodiles dig burrows into wetlands or riverbanks, giving them access to both water and a protected resting spot.
Burrows offer a major advantage: temperature stability. While surface temperatures can swing dramatically between day and night, underground spaces stay relatively constant. This insulation keeps burrowing animals cooler in summer heat and warmer during frigid nights, reducing the energy they burn while sleeping. For small mammals heading into winter hibernation, this matters even more. Dormice, for instance, build hibernation nests from bracken, pine needles, and leaf litter, often on slopes rather than flat ground to prevent flooding during long months of dormancy.
Treetop Nests and Platforms
Many birds build nests that double as sleeping sites, but the most impressive tree sleepers are the great apes. Chimpanzees construct fresh sleeping platforms in the treetops almost every night, bending and weaving branches into a sturdy bed. At one study site in Senegal, the average nest sat about 8 meters (26 feet) off the ground, with 87% of nests built between 5 and 15 meters high. Chimpanzees are selective about their building materials too, favoring certain tree species with flexible, sturdy branches. At that site, they used at least 37 different tree species, but concentrated nearly three-quarters of their nests in just eight preferred types.
Height serves a clear purpose: it puts distance between the sleeper and ground-based predators. Gorillas also build nests, though they more frequently construct them on the ground given their larger body size. For both species, building a new nest each evening likely helps avoid parasites that accumulate in reused bedding.
Sleeping on the Water’s Surface
Dolphins and whales face a unique problem. They need to breathe air, so they can never fully lose consciousness underwater. Their solution is to sleep with only half the brain at a time. This is called unihemispheric sleep, and it accounts for 70% to 90% of all sleep time in the cetaceans that have been studied. One brain hemisphere produces the slow waves of deep sleep while the other stays alert enough to control swimming and surfacing for air.
During this half-brain sleep, the eye connected to the awake hemisphere stays open 95% to 98% of the time. Bottlenose dolphins typically sleep while floating at the surface or swimming slowly. At least one dolphin has been observed sleeping while resting motionless on the bottom of a pool, but surface rest is far more common.
Fur seals take a hybrid approach. On land, they sleep with both brain hemispheres at once, like most mammals. But in the water, they switch to half-brain sleep. They float on their sides at the surface, paddling with one front flipper to stay stable while holding the other three flippers above the waterline. The awake hemisphere controls the paddling flipper, while the sleeping hemisphere rests on the opposite side. This keeps them breathing, balanced, and alert enough to watch for predators with one open eye.
Sleeping in Mid-Air
Great frigatebirds spend up to 10 days continuously soaring over the ocean without landing, and they do sleep during those marathon flights. Researchers who attached brain-activity recorders to frigatebirds confirmed that the birds sleep while airborne, using both half-brain sleep and full bilateral sleep where both hemispheres shut down simultaneously.
The catch is that they barely sleep at all while flying. Frigatebirds averaged just 42 minutes of sleep per day in flight, only about 7.4% of the sleep they get on land. They slept mostly while circling in rising air currents, with the awake eye facing the direction of flight so they could watch where they were going. Sleep bouts in the air were shorter and lighter than on land, suggesting that the demands of navigation and vigilance keep them from truly resting until they return to solid ground.
Sleeping While Standing
Horses, elephants, and several other large herbivores can sleep on their feet. Horses manage this through a “stay apparatus,” a system of tendons, ligaments, and muscles that locks the leg joints in place without requiring active muscular effort. This lets a horse doze upright for hours without collapsing from fatigue. Standing sleep is lighter, though. When horses need deep REM sleep, they do lie down, typically for short periods.
Giraffes take minimal sleep to an extreme. In the wild, they sleep as little as 30 minutes per day, almost always standing up to stay ready for predators on the open savanna. When giraffes do enter REM sleep, they lose the ability to support their own heads, so they curl their long necks around and rest their heads on their own rumps. In zoos, where predation pressure disappears, giraffes lie down more freely and may sleep up to six hours a day, a dramatic difference that shows how much predation shapes sleep behavior.
Protective Sleeping Strategies
Some animals don’t just pick a safe location. They actively build defenses around themselves before falling asleep. Parrotfish secrete a bubble of mucus that envelops their entire body like a sleeping bag. This slimy cocoon masks their scent from nocturnal predators and creates a physical barrier against parasites that would otherwise latch onto them while they rest motionless on the reef.
Sea otters take a different approach, wrapping themselves in kelp fronds before sleeping on their backs at the ocean surface. The kelp anchors them in place and prevents them from drifting into open water or onto shore while they sleep. They also frequently hold hands with other otters in their group, forming floating rafts that keep everyone together.
Why Many Animals Sleep in Groups
Communal sleeping is widespread across birds and mammals, and predator avoidance is the primary driver. By roosting or denning together, animals benefit from more eyes and ears detecting threats, which means each individual can afford to sleep more deeply. Research comparing bird species with high versus low predation risk found that species facing little danger from predators formed significantly smaller groups and were more likely to rest alone. About 35% of bird species with negligible predation risk included solitary resters, a pattern almost absent in heavily hunted species.
Group sleeping also conserves heat. Bats cluster together in caves, emperor penguins huddle in tight formations on Antarctic ice, and even small rodents pile into shared nests during cold months. The energy savings from shared body heat can be the difference between surviving a harsh winter and starving, especially for small-bodied animals that lose heat quickly.
What Shapes Where Animals Sleep
Three factors drive nearly every animal’s choice of sleeping site: predation risk, thermoregulation, and breathing access. Prey animals gravitate toward enclosed, elevated, or hard-to-reach spots. Cold-blooded and small-bodied animals prioritize temperature-stable microhabitats like burrows or insulated nests. And aquatic mammals have evolved the remarkable ability to sleep with half a brain specifically to keep breathing without interruption.
Captivity reveals just how much predation pressure matters. Giraffes that sleep 30 minutes in the wild sleep six hours in a zoo. Chimpanzees in sanctuaries sometimes skip nest-building altogether. When the threat disappears, animals sleep longer, more deeply, and in places they would never risk in the wild.