Bats
Brown bats are often said to be one of the sleepiest animals — they have been recorded sleeping almost 20 hours a day. However, their sleepy rep comes from lab studies done on captive bats in 1969, and the studies hardly mimicked the bats’ natural environment. But even if the duration of their sleep isn’t that unusual, bats are still notable for sleeping upside down.
They hang from cave ceilings, tree branches, or other surfaces thanks to specialized tendons in their feet that allow them to grip tightly while still keeping their legs relaxed. Sleeping upside down helps bats defend against predators: Unlike birds, bats can’t just flap their wings to take flight, and they use gravity to take off. Hanging upside down is their pre-flight pose, so if a predator attacks while they’re snoozing, they’ll drop down and instinctively fly off before they’ve even fully woken up.
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