Busting the myths of animal sleep

Sleeping animals

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Most of us can agree about counting sheep, or letting sleeping dogs lie, but the facts about forty winks in the animal kingdom are still subject to debate.

The mystery of sleep remains a source of fascination for biologists but at the last count they had only studied it in around 50 of the 60,000 or so vertebrates on Earth.

Experts and filmmakers have teamed up to try to learn more in the BBC Four documentary Animals at Night: Sleepover at the Zoo.

As technology to monitor behaviour and brain activity advances, new discoveries are being made every year and long-held theories can be turned on their heads.

So how much do you really know about animal sleep?

Sloths do not sleep the most

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Despite their reputation, sloths are not the animal kingdom's biggest fans of a snooze.

They might be slow-moving but a study of wild sloths found the animals sleep for around 9 to 10 hours per day.

Koalas get 14.5 hours shut-eye but little brown bats are the longest sleepers on record so far, after they were clocked sleeping for 20 hours.

Meanwhile, the animals that need the least sleep are the giants that graze. Giraffes and elephants have just three to four hours a night.

Not all cats nap 90% of the day

The rumour that lions always lie around for more than 20 hours out of 24 is another pub quiz favourite.

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The big cats, like their small domestic cousins, are cathemeral, meaning they take short, sporadic naps regardless of the amount of light so they can take advantage of a good meal at any time.

But in a study of lions in South Africa, researchers found the cats were only resting for around 14 hours of their day.

The scientists suggested that the lions in Addo National Park were more active because they didn't have an established pride, so spent more time on the lookout for danger from competing lions or trampling elephants.

Whales can dream

While they can't directly measure whether an animal dreams, scientists can measure whether an animal experiences rapid eye movement, or REM.

The REM phase is the deepest stage of sleep, when muscles move the least and humans dream.

This lack of muscle movement is a problem for marine mammals however, because they have to return to the surface of the water regularly to breathe.

Whales are known to partially sleep with one half of their brains at a time, known as unihemispheric sleep, so that they are still able to make essential movements.

But to complicate the issue, recent evidence suggests whales could also enter deep REM sleep, even if only for short periods of time.

Dr Patrick Miller from the University of St Andrews was part of a team that recorded video footage of a motionless group of sperm whales resting with their heads at, or near, the surface of the sea.

The whales remained unusually still until they were accidentally disturbed by a boat and Dr Miller suggests this is evidence of a REM sleep phase in the mammals.

Albatrosses do not sleep while flying

Wandering albatross Albatrosses take naps on the waves

Wandering albatrosses are famed for their long distance flights, covering thousands of kilometres over open sea before returning to land.

Their journeys take so long that it was assumed they must sleep while flying.

A recent study revealed how the birds can stay aloft without any mechanical effort thanks to their unique wing joints, lending weight to the theory that they can sleep on the wing.

But this specialised flying-style does not complement sleep according to Dr Niels Rattenborg from the Max Planck Institute for Ornithology.

"When compared to soaring up in the sky with nothing to run into, dynamic soaring near the waves may be more cognitively challenging and therefore result in a greater need for sleep," he said.

"This might explain why most tracking studies have found that albatrosses usually stop and float on the water for several hours at night."

The biologist's studies of sleep in birds suggest that rather than taking an in-flight nap, some species have evolved to go without sleep altogether when they have a long journey to complete.

"Our recent discovery that pectoral sandpipers can greatly reduce their time spent sleeping during the Arctic summer breeding season without experiencing deficits in performance raises the possibility that at least some seabirds may have evolved a similar ability to suspend sleep during long flights at sea."

Sharks are not all restless

Animals at Night: Sleepover at the Zoo

Liz Bonnin standing next to a lion in the zoo

Animals at Night: Sleepover at the Zoo broadcasts on BBC Four, Monday 3 March 2014 at 21:00 GMT.

Instead of popping up to the surface like whales, sharks respire by forcing oxygenated water over their gill slits.

Certain species, such as nurse sharks, can do this by sucking water through their mouths or special openings on their heads known as spiracles.

These sharks have been seen lying motionless on the sea bed, leading to theories that they could be sleeping, but other species have to keep swimming forwards relentlessly in order to force the water over their gills.

This continuous motion might not sound very restful but a study of spiny dogfish sharks found the swimming activity was actually controlled by the spinal cord, not the brain.

Shark scientist Prof R. Aidan Martin suggested this could be evidence that sharks can be unconscious, even though they are moving, which gives them the opportunity to rest their brains.

Join BBC Nature on Facebook and Twitter @BBCNature.

Animals at Night: Sleepover at the Zoo broadcasts on BBC Four, Monday 3 March 2014 at 21:00 GMT.

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