Manger has been studying sleeping animals for almost two decades. He began with, of all things, the platypus, which turned out to get more rapid eye movement (REM) sleep—the type in which dreams occur—than any other animal. From that quirky start, Manger went on to study sleeping dolphins, whales, hippos, echidnas, cats, and antelopes.

But most of that research involved captive animals, which enjoy plentiful food and an absence of predators. For those reasons, they sleep much more than their wild cousins. For example, in the 1980s, scientists found that captive three-toed sloths sleep for 16 hours a day—a result that earned them a reputation for, well, sloth. In 2008, when other researchers recorded the brain activity of wild sloths, he found that they only sleep for 10 hours a day. Manger also wanted to take the science of sleep into the wild. And he started with elephants.

As a very rough rule, and for reasons that are still unclear, bigger mammal species tend to sleep less than smaller ones. Captive elephants reportedly sleep for just 3 to 7 hours a day, but for their size, you’d expect them to get even less. Proving that is hard, though. Elephants can sleep standing up, so it’s difficult to eyeball whether they’re awake or asleep, especially if you’re tracking them through the bush at night. Brain activity sensors would give better answers, but an elephant’s anatomy makes it exceedingly risky to surgically implant such devices. So Manger and Gravett settled for the actiwatches instead. They also fitted satellite collars onto the animals to track their whereabouts later.

As Manger suspected, the two females got just over two hours of sleep a night, and even that isn’t unbroken—it occurs in four to five bursts, spread throughout the night. On most nights, they slept standing up; on some, they lay down. They weren’t fussy about their choice of sleeping spots, and the distance they traveled during the day had no effect on how long they slept.

“Under natural conditions, animals should show shorter sleep times than in the lab, where food is always available and there are no predators,” says Isabella Capellini from the University of Hull. “It's very nice that we start getting estimates of sleep outside the lab.”

Of course, Gravett and Manger only studied two elephants, and both were adult matriarchs—the individuals who bear the burden of leading the herd. It’s possible that males, youngsters, or lower-ranked females might get more shut-eye, but it’s at least clear that in this particular demographic, sleep time is superlatively low.

The gray whale is much larger than an elephant, but still sleeps for nine hours a day. A dolphin can sleep with one half of its brain at a time, allowing it to stay constantly alert for days at a stretch—but each half still gets at least four daily hours. Oddly, the next shortest sleeper seems to be the domestic horse, which gets just three hours a day, followed by the domestic pony, which gets slightly more. For now, it appears that the African savannah elephant is the shortest sleeper around. (The longest seems to be the little brown bat, which gets an incredible 19 hours a day.)