All kinds of tired

Donkeys sleep about three out of each 24 hours. Certain reef fish spend the night moving their fins as if swimming in their sleep. Some biologists argue that all animals sleep in some form or another. But identifying sleep can get complicated. Insects have brain architecture so different from humans’, for example, that electrophysiological recordings during “sleep” won’t match human patterns. The real problem may be that researchers haven’t agreed on what sleep does for people, so it’s hard to agree on the animal equivalent. Studying animal sleep, though, offers the prospect of discerning evolutionary patterns in sleep pointing to some ancient function. — Susan Milius

Fruit fly 8–10 hours of inactivity each day
Lab fruit flies droop into less-responsive, sleeplike periods mostly at night. If deprived of these quiet bouts, flies spend extra time stationary later, as if catching up. Caffeine keeps them awake, and antihistamines increase downtime. Studies haven’t found REM patterns, but brain activity does shift during the droops.

White-crowned sparrow 3–8 hours (depends on season)
During migration season, white-crowned sparrows perplex researchers with the birds’ apparent power to cheat on sleep. Birds get not quite 40 percent as much sleep as usual, with drops in both slow-wave and REM sleep. Yet the birds don’t get stupid in performance tests.

Platypus 14 hours
From an ancient mammal lineage, the platypus shows REM activity in its brainstem but not simultaneously in its forebrain, as many other mammals do. The platypus forebrain shows non-REM sleeplike activity during this time, though, and the REM session lasts long relative to other mammals.

Armadillo 17–20 hours
Armadillos appear to be prodigious sleepers. The nine-banded armadillo has been clocked sleeping more than 17 hours, and the giant South American armadillo 20 hours, among the longest stretches recorded.

Lab rat 11–14 hours
A lab rat perishes when marooned for weeks on a disk that tips it into water when the rat dozes off. Rats survive other kinds of sleep-deprivation tests, though, inspiring a debate on whether it’s sleep loss or a side effect that is fatal.

Three-toed sloth 9.6 hours (measured in the wild)
Brown-throated three-toed sloths wearing portable recorders became the first free-living animals to have their electroencephalograms studied. Out in the forest, the adult female sloths slept some six hours less in a 24-hour period than captive sloths did. The wild animals’ EEGs showed about two hours in REM sleep.

Giraffe 4–5 hours
Giraffes sleep only a few hours out of 24, but lions, which prey on giraffes, have been clocked snoozing around 13 hours a day. Some reseachers have noted a trend toward less sleep in species that, like giraffes, rest in more exposed situations in which they might be more vulnerable to predators.

Bottlenosed dolphin 4 hours (in each brain half)
Bottlenosed dolphins and other cetaceans studied so far show typical mammalian slow-wave brain activity during sleep but in only one brain hemisphere at a time. Researchers have documented little, if any, REM sleep in cetaceans.

From top: Jane Burton/Photo Researchers; Scott Bauer/USDA-ARS; Wolfgang Wander; Philip Bethge; Leppyone/flickr; Jean-Etienne Poirrier; Praziquantel/flickr; Rob hooft; Just Taken Pics/flickr

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.