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All mammals, including humans, share some common characteristics, and sleep is one of them. Sleep cycles of REM and non-REM sleep are another common denominator, according to the National Sleep Institute. However, where there are differences between mammals is in the amount of sleep needed and the preferred sleeping posture. Humans lie down and a horse stands up, but three mammals have good reasons for sleeping upside down.
Anatomy, physiology, environment and environmental adaptations all affect the way different species sleep. Animal sleeping practices are diverse: dolphins shut off only half the brain during sleep and sleep while swimming; cows sleep with their eyes open; a gorilla likes to make a cozy nest, while moles and rabbits burrow underground. Each mammal has a sleeping style that fits its environment. The mammals that sleep upside down adopt what seems to humans an uncomfortable position, for safety and because they can't function in any other position.
Bats are probably the mammals best known for their upside-down sleeping position. One reason for this is that it makes the bat less visible to predators. There is also a good physiological reason for hanging from the ceiling: a bat's wings aren't powerful enough to lift him into flight from an upright, standing position. By hanging he can fall into flight mode at a moment's notice. Bats spend a substantial part of the day sleeping: they pop out at dusk for two to three hours, then return home for a nap before appearing again for a short time before dawn.
The sloth has a reputation for slowness and sleep. Like the bat, this rain forest tree-dweller spends at least 18 hours of his day sleeping upside down. His long arms and claws enable him to lock around a branch and hang there. Pregnant sloths even give birth upside down, a feat that the bat mother can't match. The nocturnal sloth sleeps with his head tucked between his arms and chest, and keeps his feet close together. This posture provides camouflage by making him look like a tree branch.
The manatee, a marine mammal native to Florida and the Caribbean waters, might die if he didn't sleep upside down. Typically, this large herbivorous mammal spends his time in fairly shallow waters with a depth of about 6 to 8 feet. Here he grazes on sea grass and algae. He spends about 12 hours sleeping, but because he is a mammal and doesn't have gills for breathing under water, he may suspend himself upside down close to the water's surface so he can take air in through his nostrils. The manatee doesn't always sleep in this position; sometimes he sleeps on his back on the sea bed.
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