Strange sleeping habits

If you’re feeling sleepy after the holidays, here are some truly bizarre facts about animal sleep to wake you up

Dolphins sleep with one eye open

Some animals never leave the water and have no choice but to sleep while they swim. Dolphins (as well as ducks, iguanas and some whale species) rejuvenate using unihemispheric sleep, where one half of their brain shuts down and rests while the other stays awake and alert.

Always having at least half a brain taking in the surroundings keeps pods of dolphins out of trouble and means they can still swim and surface when they need oxygen. Sometimes they enter a deep sleep and stay at the surface, something referred to as ‘logging’ because of the way the pods float.

Most young animals rest more than adults, but calves spend at least the first month of their lives without sleep. Constantly swimming puts calves in a better position should they need to escape, and the movement maintains their body temperature until they’ve built up enough blubber to insulate them.


Common swifts have to sleep on the wing

Many migratory birds spend huge stretches of time on the wing without landing, but the common swift takes it to the extreme.

These little birds fly between their African wintering grounds and their breeding grounds further north, spending up to ten months in the air without landing. Warm air during the day helps them to travel with less effort, and their aerodynamic wings are highly efficient, but this incredible feat still raises a question: when do they sleep?

Researchers fitted trackers to swifts to monitor their movements and found that they climbed high in the air at dawn and dusk. After gaining several kilometres, the birds began a slow glide down again. The descent lasted about half an hour, and the scientists think this could give them enough time for a power-nap.


Parrotfish tuck themselves in at night

Campers and holidaymakers will be familiar with the idea of wiggling into a sleeping bag as the day draws to a close, but it’s unlikely that they made it themselves.

Before going to sleep each night, members of several parrotfish species enclose themselves in a case constructed using their own mucus secretions. This cocoon acts like a mosquito net to keep away parasites roaming the night in search of blood, as the structure allows water in but is too fine for them to fit through.

The mucus probably also masks the fish’s scent so predators don’t discover them, and it could act as a warning system as the fish notices any disturbance to their sleeping bag before the danger gets too close.


There are even more strange sleepers in Issue 54 of World of Animals, available now in store and online