Do all animals need to sleep?

27 June 2024

If we sleep too little, we become unfocused and tetchy and will eventually fall ill. Among humans, it isn’t just parents of newborn children who have a tale or two to tell about this. But what about the animal kingdom? Do all animals need to sleep?

The short answer to this question is yes! No animals can do without sleep, be they great or small. But when, for how long and in which situations they sleep differs drastically between different types of fauna.

One of the few creatures that can go without sleep, at least for periods, is Caenorhabditis elegans. This nematode sleeps as a larva, but as an adult worm it only naps now and then, instead of regularly like we do. And it sleeps as soon as it gets stressed – when it’s hungry or too hot or too cold, for example. “You always need sleep when you’ve learned something new, when something’s changed in your nervous system,” says sleep researcher Albrecht Vorster. An adult nematode doesn’t have much more learning capacity, which is why it probably no longer needs to sleep, Vorster told the journal “Spektrum der Wissenschaft”: “Even if you can argue the point a bit in the case of the nematode, everything that has a brain seems to need to sleep.”

But even brainless animals take a nap now and again. The mangrove jellyfish Cassiopea andromeda, for example, has no brain and doesn’t even have a central nervous system. And yet, the jellyfish ticks the key boxes for sleep: In this state, it demonstrates a delayed reaction to stimuli. It can be woken up comparatively quickly, however – unlike animals that are in hibernation. And if something has prevented it from sleeping, it makes up for what it has missed the next time it naps, sleeping longer and more deeply.

For our distant relatives in the sea, nature has come up with a special sleep strategy: Dolphins regularly have to come to the surface to breathe; otherwise, these marine mammals would drown. So, Flipper’s kin sleeps in shifts, namely with one hemisphere of the brain at a time, and only closes one eye. The other half stays awake, taking over the swimming and surfacing functions. This is a remarkable achievement, as one hemisphere of the brain has to control the entire body, whereas in our case, both halves always share the work. So as not to overwhelm their waking half with this task, dolphins often look for quiet bays to sleep and usually stay just below the surface. The largest dolphin species – the orca – also sleeps according to this shift principle and gets by with just 90 minutes per day.

Being half asleep is also very common high up in the sky: Swifts, for example, fly thousands of kilometres to their winter quarters in Africa without ever touching the ground. They have to get the sleep they need in the air while gliding. Researchers suspect that in birds, as in dolphins, only one half of the brain sleeps, while the other takes the helm. And because the waking hemisphere would be overwhelmed with more complex flight manoeuvres, the periods of sleep are extremely short: On average, a swift’s power nap lasts ten seconds.

Frigate birds also spend weeks in the air on their migrations and allow themselves up to six minutes of sleep at a time during the flight. To allow them to sleep, they let themselves be carried upwards in circular movements by updrafts to save energy. They also sleep with one hemisphere of the brain, and the eye facing in the direction of flight stays open. In this way, they can avoid colliding with other fellow travellers.

Ducks, on the other hand, keep one eye open while sleeping to check whether a hungry fox or marten might be sneaking up on them. If they should pass the night in a group, the ducks on the perimeter spend about a third of the night with one eye open. From time to time, they turn around to give their other hemisphere some sleep as well. During the night, the ducks in the group change positions from time to time, so that the guards can also close both their eyes for a while.

In general, the sleep routines in the animal kingdom have a lot to do with what any given animal eats and whether it is a prize delicacy for other animals. Herbivores that can’t retreat into burrows must be constantly on their guard. While lions can treat themselves to 13 hours of sleep a day, horses, for example, only sleep a total of three to five hours. They do this only for short periods, and only rarely do they lie down. They will doze off for another three hours during the day.

In general, herbivores usually sleep much less than carnivores. They don’t have the privilege of being able to snuggle down anywhere. They have to eat all day and often cover longer distances. This is because grass and leaves provide less energy than meat. Elephants therefore only doze for three to four hours a day.

Tree-dwelling herbivores such as sloths, on the other hand, rely on the opposite principle: They live in energy-saving mode. Sloths sleep through about three quarters of the day and move only in slow motion when awake. Because they feed almost exclusively on hard-to-digest leaves, they have one of the slowest metabolisms of all mammals. Meaning that they have to manage their energy very well. However, the proverbial slowness of the sloths also offers them a real advantage over more nimble tree dwellers: Their leisurely movements are hardly recognisable to birds of prey flying high above in the air.

Incidentally, the sleepyhead record among mammals is held by koalas. These small marsupials with the fluffy ears spend a maximum of four hours of the day on their feet. As with the sloths, this also has to do with their favourite food: They feed exclusively on low-energy and particularly hard-to-digest eucalyptus leaves. If koalas have to cope with stress phases with fewer than 18 hours of sleep per day, these little denizens of Australia will die of exhaustion. Persistent insomnia can therefore have serious consequences, and that not only for new parents.