Provided the average person hasn't sprung a leak, they have between five and seven liters of blood sloshing around their body at any given time. All this red stuff is subject to one of the supreme rulers of the universe: gravity. And at no time is this more evident than when you dangle upside down or hang your head off of the bed because you’re being goofy. All that blood begins rushing towards your noggin, putting serious pressure on your brain and resulting in a headache. Were you to remain in that position for a long enough time, you'd pass out. Suffice it to say, the human body wasn't designd for an upside-down life.
How then, can animals like bats — which are, like us, mammals — sleep upside down for 15-20 hours a day without their heads bursting like a rubber band-strapped watermelon? And how do their feet manage to hold a grip when they dip into sleep and their muscles relax? The answer, which lies in a couple of biological tricks gifted to them by evolution, speaks to the nature of human sleep.
When bats close up shop after a night of foraging for fruit or catching insects, they end up going to roost in secluded spots in colonies of millions, small groups of hundreds or familial bands of a few. And they're all hanging upside down, whether they’re testing the integrity of some poor tree or getting cozy in a farmer's barn.
Bats hang for a couple of reasons. Being able to dangle from their feet, for example, gives them more options when looking for places to roost (the ceilings of caves being the most obvious). And napping in these hard-to-reach, isolated spots helps them avoid predators, mainly birds of prey, that are active during the day when bats are sleeping and therefore at their most vulnerable.
Hanging upside down is also the only way that these flying mammals can take off. They're not able to get enough lift to launch from the ground, nor do they have serviceable enough hind legs to get a running start for takeoff. But, “by simply dropping down from a cave ceiling, or tree branch, a bat can quickly and effectively achieve flight,” Phil Garofolo of the Organization for Bat Conservation told Van Winkle’s.
And while we humans have quarts of blood flowing through our heavy, fleshy bodies, bats, by comparison have hardly any. Your average homo sapien weighs between 150 to 180 pounds, where even the largest bats on earth (the black flying fox and golden crowned flying fox of Australia and the Philippines, respectively) only clock in at about 2.5 pounds. Most bat species weigh mere ounces. Because they pack so few pounds and such little blood, gravity doesn’t affect their circulation, thus freeing them from the tremendous vice-like pressure that would afflict any large animal that hangs upside down.
Okay, now that the rush-of-blood-to-the-head-mystery is solved, how do bats achieve the Houdini-like feat of clinging to a cliff ceiling or tree branch after they’ve fallen asleep? An olympic gymnast can probably droop downwards off a high bar, holding on by the crooks of their knees and dangle there in a relatively relaxed position. But, the minute the gymnast loses consciousness — either because of excess blood flow to the head, or a miraculous ability to fall asleep in that position — their relaxed muscles would loosen their grip, gravity would take over and down would come baby.
Evolution by natural selection has put together a winning combination for bats to overcome this problem. Similar to the way a horse's leg muscles stiffen to let it sleep standing up, bats have developed a locking system of sorts that keeps their claws and feet clenched. The tendons in bats’ feet travel through a rough sheath-like structure in their ankles that closes tight upon the tendon when the ends of it are pulled. Just like a chinese finger trap.
And like a finger trap, when one of these critters alights upon a tree limb or rock crag and hangs down, the pull of their body weight downward stretches that sheath and locks their feet in place, letting them relax their muscles and exert zero energy.
“Whereas our hands are naturally open at rest, a bat expels no extra energy keeping their claws closed,” said Garofalo, who added, “to release from a wall, a bat actually has to flex their muscles.” This system works so well, in fact, that it can cause issues in old age or death. Garofalo explained that “as a bat gets older, arthritis can become an issue — rather than not being able to cling to ceilings, they actually get stuck easier as their fingers become harder to open.” And if they die while hanging, they’ll likely continue to do so long afterwards.
What all of this unique sleeping behavior ultimately highlights, however, is the universal need for sleep in the animal kingdom, across all creatures. They all make it work. Whales bob near the surface, resting one half of their brain at a time so they can still maintain breathing function (they can't breathe underwater); frigate birds, which travel long distances overseas and cannot swim, catch sleep for minutes at a time while gliding thousands of feet in the air; and humans and apes build physically and mentally comfortable beds in order to give our complex, sleep-hungry brains the deep sleep they need to function properly. Sleep is essential to our evolution and so is evolution to our sleep.