Do animals sleep well? They sleep standing up, hanging upside down, with their eyes open...

Sleeping is an indispensable behavior for us humans and many animals. About one-third of a person's life is spent sleeping, whether sleeping on your back, side or stomach. Human sleeping positions are varied. Looking at the entire animal kingdom, there are even more bizarre sleeping positions competing for beauty.

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#1 Sleeping Standing Up

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In the long process of evolution, many large herbivores have evolved the ability to sleep standing up, such as horses, African elephants, giraffes, etc. They can sleep standing up so that they can quickly escape when they encounter danger. But if they want to get a thorough rest, many species will still lie down or lie on the ground to rest at certain times. Some birds have taken standing sleeping to the extreme. Not only do they stand on one leg, but they can also get a full rest without lying down. For example, wading birds such as red-crowned cranes and flamingos all adopt this sleeping position. The long legs and the standing sleeping posture make them less picky about the place to sleep, which is very suitable for water birds such as red-crowned cranes that live in swamp wetlands.

#2 Sleeping on a tree

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In order to avoid predators, some animals choose to live in trees, and sloths are one of them. Sloths are known for their slow movements and spend most of their lives in trees, so they have developed the ability to sleep in trees. When they sleep, they usually use their limbs to hook onto branches, so that they sleep in a hanging position. This sleeping position allows sloths to hang steadily on branches and avoid accidentally falling off.

#3 Sleeping Upside Down

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Bats are the largest group of mammals besides rodents, and they are also the only mammals that can truly fly. Bats generally live in caves, tree holes, etc. They have very weak walking ability, and when they land, they hang upside down on tree branches, eaves, cave ceilings, etc. Therefore, they sleep upside down, and they also wrap themselves with their wings, like a blanket.

#4 Sleep with eyes open

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An obvious characteristic of us humans when we sleep is that our eyes are closed, but in fact many animals cannot close their eyes. In the entire animal kingdom, only four groups of animals, namely amphibians (such as frogs), reptiles (such as turtles), birds and mammals (i.e. beasts), have evolved eyelids (and some reptiles such as snakes do not have them), so they can close their eyes. Other animals cannot close their eyes, so they can only sleep with their eyes open. Among the animals that can close their eyes, some species also open their eyes when they sleep, such as owls. Owls are a general term for birds in the order Strigiformes. They sleep during the day and hunt at night. They are typical night owls. In order to stay alert, adult owls often open one eye and close the other when they sleep.

#5 Floating hand in hand

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Sea otters (tǎ) are mammals that are used to living on water. They complete almost all activities in the water, and even sleep while floating on the water. When they want to sleep, they will choose a place where seaweed grows and wrap the seaweed around their bodies so that they are not afraid of being washed away by wind and waves while sleeping. They also like to sleep in groups, and when there are companions around, they will sleep together hand in hand.

#6 Sleeping in Vertical Suspension

Sperm whales are mammals like sea otters, but unlike sea otters, which still retain their mammalian appearance, sperm whales have fully adapted to marine life and have become "fish-shaped." The sleeping posture of sperm whales is quite interesting. They sleep in the seawater with their heads upright, like thick logs.

#7 Sleeping While Awake

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Dolphins belong to the family Delphinidae of the order Cetacea, and therefore are also whales. They also cannot breathe in water and need to surface to breathe air. Unlike sperm whales that can dive for up to one to two hours, dolphins can only dive continuously for a few minutes to more than ten minutes. In order to avoid drowning while sleeping, their breathing needs to be completely controlled by their own consciousness, and they cannot breathe automatically in an unconscious state like humans, so dolphins cannot fall asleep completely like humans. When dolphins sleep, only half of their brain rests, while the other half of the brain remains conscious and alert, that is, they are in a state of both sleeping and waking. Interestingly, when the left side of the dolphin's brain is resting, it closes the right eye, and vice versa.

#8 Sleeping with a blister wrapped around your face

Parrot fish are brightly colored fish that live among corals. They have conspicuous and hard teeth and feed on corals. Some species of parrot fish will use mucus secreted by special glands in their gills to form a transparent "bubble" to wrap themselves in before going to sleep, so that they can avoid being harassed by parasites while sleeping, and also cover their smell to prevent predators such as moray eels and sharks from discovering them.

#9 Sleeping While Flying

Swifts are migratory birds with strong flying ability. They migrate tens of thousands of kilometers every year to and from their breeding grounds and wintering grounds. Due to structural problems, swifts' feet are not suitable for walking on the ground, so they spend most of their time flying in the air. Studies have found that swifts can stay in the air for several months without landing, and have the ability to sleep in flight. When swifts want to sleep, they will first fly to a high place, and then sleep while gliding, achieving the magical sleeping posture of sleeping while flying.

Source: Chongqing Science and Technology Museum

Author: Cheng Chaoyuan, PhD, Institute of Zoology, Chinese Academy of Sciences, and Associate Researcher, Hainan University

Review experts: Huang He, Li Chunli, Xu Xiaoping

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