Do animals sleep? Yes, but proving it takes more than a look. A sleeping ostrich sits still with its eyes open and its neck upright, so it looks alert. Yet recordings show that in its deepest sleep, its brain waves are slow and strong. A sleeping animal does not always look the way you would expect, which is why scientists use tests.

One way to be sure is to read the brain. Scientists put helmets with electrodes on six ostriches. The helmets recorded body temperature, brain signals, eye movements and neck muscle contractions. The readings clearly showed when the birds were asleep.

Another way is to test how an animal reacts. Sperm whales float upright, almost still, with their heads at or just below the surface. Researchers brought a boat near some floating whales, and the whales did not react, even though both eyes could see the boat, until it accidentally touched one. Octopuses were shown crab pictures and vibrations. Their delayed response, or total lack of reaction, suggested they were asleep.

An octopus has two kinds of sleep. In quiet sleep, it is pale and still, and its pupils narrow to slits. In active sleep, its skin flashes changing colors and textures, and its eyes, arms and suckers move. Active sleep is rare and brief. It comes about every half hour, lasts roughly 40 seconds, and fills less than 1 percent of an octopus's day.

Brain readings can also surprise us. Bearded dragons are reptiles, and they show both slow-wave sleep and REM sleep, two kinds of sleep. Scientists once thought these kinds of sleep belonged only to mammals and birds. Since a lizard, a bird and a mammal all have them, the kinds of sleep probably existed in their common ancestor.

Asleep can even mean half asleep. A dolphin sleeps with one half of its brain resting while the other half stays awake. Some birds do this too. A bird that sleeps with half its brain closes only the eye on the sleeping side.

Sleeping in the air is the hardest thing to check. Alpine swifts may hardly ever land. Scientists tagged six of them with small recorders of light and movement, and the data showed the birds stayed in the air for about 200 days. Great frigatebirds were tracked on flights of up to ten days and 3000 kilometers. If these birds sleep on such trips, they must do it while flying.

Not every deep rest counts as sleep. Hibernation is not just a long sleep. It is better understood as a state called torpor, a state of very low energy use. Bats save energy by entering torpor. A black bear in winter sleep lets its body temperature fall only about five degrees Celsius.

Where you test sleep can change the answer. To measure sleep in wild sloths, scientists fitted them with a small, light cap of brain sensors, plus a radio collar. The wild sloths slept only about 9.6 hours a day. Older studies of sloths in captivity had said about 16 hours. Captive sloths may have been more relaxed, with no predators and food always supplied. The scientists warn that measuring brain signals only in captivity can lead to the wrong conclusions about wild animals.

Words to know

• electrodes: sensor pads
• torpor: deep rest
• upright: straight up
• reptiles: lizards and snakes