Do all animals sleep?

It’s no secret that we humans love our sleep. Without it, brain function rapidly degenerates, and we’re not the only ones this affects.

From fruit flies to fruit bats, creatures across the animal kingdom take time out to switch off. Sloths, giant armadillos and opossums can get up to 20 hours of shut-eye in a day. But sleep is a bit of an enigma; we don’t fully understand why it happens, and whether all animals need to sleep is not yet known.

Sleep is defined as a temporary state of immobility and reduced responsiveness. This is in contrast to
a coma (which is not temporary), or resting (where we’re still mobile and responsive).

Simple animals like nematode worms and cockroaches show signs of sleep, as do honeybees and zebrafish. Even dolphins catch some ZZZs underwater, putting just half of their brain to sleep and keeping the other half active so that they can continue to move and keep one eye on their surroundings. This remarkable form of napping is called unihemispheric sleep and has been observed in other aquatic mammals, as well as some bird species including blackbirds, chickens, mallard ducks and peregrine falcons.

Carnivores spend more time sleeping than herbivores, partly because herbivores have to spend much of their day eating. In land mammals, young animals tend to sleep for considerably longer than adults – they’re growing and developing, and that requires a lot of down time.

One of the only animals that has been reported to go without sleep is the bullfrog. More experiments are needed to definitively say that they don’t snooze, but they seem to be an exception in an otherwise very sleepy animal kingdom.