Do All Animals Need to Sleep?
From the armadillos who spend the majority of each day asleep, to the giraffes who receive just two hours (1), scientists believe that virtually all animals sleep (2). Even an animal that is one millimeter long, the caenorhabditis elegans (3), sleeps.
Sleep looks different depending on the animal, but it's usually defined as a short period of being relatively still (4), with reduced reactions to the outside world. Spending time in this vulnerable state can be dangerous and it takes time away from reproducing or hunting for food (5). Sleeping must be important, or we wouldn't bother to do it. Scientists hope that sleep research on animals might help shine light on why we sleep.
Most land mammals (6) show distinct periods of restorative slow wave sleep (SWS) alternating with rapid eye movement (REM) sleep, which is typically when dreams occur.
Mammals' sleep needs vary depending on whether they are predator or prey (7). Predators tend to sleep longer hours, but these needs can be affected by diet and other factors (8). Animals that hibernate, such as bears, tend to sleep for different lengths of time depending on the climate and season.
Animals that are lower on the food chain tend to only sleep long hours if they are able to find a protected sleeping space. The largest land mammal, the wild elephant (9), sleeps just two to three hours a night in total, mostly in the form of short naps while standing up. If needed, wild elephants are able to go up to 46 hours without sleeping.
Smaller omnivores like armadillos may sleep up to 20 hours, possibly because they find it easier to hide while sleeping. Three-toed sloths have also been reported to sleep around 16 hours in captivity, though sloths in the wild may only sleep 10 hours (10). Instead of sleeping all at once, sloths sleep in short stints, to leave time for foraging food in between. When surrounded by nocturnal predators (11), sloths switch to sleeping predominantly at night to avoid detection.
Dolphins and whales (12) display what's known as unihemispheric sleep, (13) in which one half of the brain remains awake. During unihemispheric sleep, marine mammals may be less responsive, but they keep swimming and can still monitor their environment to some extent. It's thought that unihemispheric sleep allows marine mammals to keep surfacing for air (14) and regulate their temperatures (15) more easily. Newborn whales and dolphins appear not to sleep for about a month after birth, but it's likely that these are simply cases of unihemispheric sleep (16).
Sleep is a little different for mammals that live on both land and in water. Walruses (17) can swim for an impressive 84 hours straight, but they prefer to do their sleeping on land. By contrast, fur seals (18) prefer to sleep in the water, where they engage in unihemispheric sleep. While sleeping in the water, elephant seal pups (19) can go up to 12 minutes without breathing.
Researchers have failed to find examples of REM sleep in many marine mammals. REM sleep is usually accompanied by a lack of muscle movement, which could be dangerous when in the water. That said, fur seals and walruses display periods of REM sleep when they choose to sleep on land.
Reptiles and Amphibians
There is little research on whether or not reptiles and amphibians sleep (20). Based on research from the 1960s, the bullfrog gained a reputation for being an animal that never sleeps. However, scientists have since broadened their definition of what sleep can look like in animals. Newer research on iguanas (21) and bearded dragons (22) suggests that reptiles do experience a sleep-like state. Certain reptiles, like crocodiles, also know how to sleep with just half their brain.
Scientists originally believed that birds were able to go for many days without sleeping. In 2013, researchers tracked a group of Alpine swift birds for 200 days (23), during which the birds didn't once stop flying. However, scientists now believe that birds might actually be sleeping while they fly (24).
Like many animals, ducks tend to sleep in groups for better protection. Among ducks sleeping in a row, those on either end sleep with one eye open (25) to watch for predators, while ducks in the more sheltered positions sleep with both eyes closed.
Birds experience both REM and non-REM sleep. One study found that birds showed more REM sleep and deeper slow wave sleep when they were allowed to sleep on a higher perch, further from predators (26).
Most rodents obtain 10 to 15 hours of sleep per day, with evidence suggesting this includes periods of REM and slow-wave sleep (27). Like humans, scientists have found that rats die (28) if they don't get a chance to sleep for a few weeks.
Scientists believe several types of fish show clear distinctions between sleep and wake. Some of these fish become completely still, while others keep their fins moving in order to breathe. Sharks and rays (29) also show periods of reduced activity, but it's unclear whether these larger fish are sleeping or simply resting.
New research has also found that jellyfish appear to sleep (30). This is important because jellyfish are one of the oldest known life forms still around today. If jellyfish sleep, it could support the theory that sleep played an important role in evolution.
Flies and crayfish (31) display many of the same brain activity patterns as humans do while they sleep. Flies and cockroaches die if they go too long without sleeping. A very rudimentary type of flatworm (32) appears to sleep on a daily basis, reinforcing the idea that sleep evolved in primitive life forms.
Interestingly, bees seem to sleep different amounts of time depending on their age and their role within the bee social structure (33).
So, Do All Animals Sleep?
So far, researchers haven't managed to find any animals that don't sleep. While sleep may look very different in a dolphin compared with a chimpanzee, it seems clear that sleep is just as important as breathing or eating to most, if not all, animals.
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