The weird and wonderful world of animal sleep

We might be capable of getting our shut-eye in all kinds of places, but nothing beats the sheer variety of sleeping habits in the broader animal kingdom.

Animals can sleep on the wing, standing up, sitting in a tree or diving in the ocean. And they don't have to have their eyes shut either.

The going consensus is that all animals need to sleep, although scientists can't be completely sure.

In the first place, it can be difficult to tell when a creature is asleep just by observing their behaviour.

Animals that look asleep to humans may actually be awake and vice versa, says John Lesku from La Trobe University, who studies sleep in animals.

The way to know for certain is by measuring an animal's brainwaves to see if they show the telltale pattern of sleep.

And then there are cases like the sea sponge that doesn't even have a brain to measure!

Still, research so far has given us some fascinating insights. 

If you think your slumber habits are weird, check out the weird ways other animals snooze and how they differ from us.  

Eyes wide shut

While we close our eyes when we're asleep, some animals such as fish and snakes sleep with both eyes open.

"They have no choice because they lack eyelids," Professor Lesku says.

But ostriches also sleep with their eyes open — despite having eyelids (in fact they have three pairs!)

It turns out these birds only keep their eyes open during slow wave (deep) sleep – likely to maintain some degree of vigilance, Professor Lesku says.

When ostriches drift into REM sleep (the lighter stage of sleep where humans have the most vivid dreams) they close their eyes.

And then there are animals that sleep with one eye open — such as dolphins.

These animals actually sleep with just half their brain so they can keep an eye out for predators while they have a kip.

Whatever sleep position works

We generally prefer to sleep lying down in bed – on our side, back or – less desirably — on our tummies.

But nothing compares to the weird and wonderful positions and locations that other animals sleep in.

The great frigate bird spends months at sea looking for food. During this time it sleeps as it flies, catching short naps as it soars on the warm air currents.

Sperm whales sleep vertically, bobbing around just below the water surface – their large heads end up at the top because they are more buoyant, so the animals look like they are standing in the water.

Meanwhile on land, cows and horses are among the animals that actually do sleep standing up, and cows even chew food while they sleep.

They do this during slow wave sleep, making it easier for them to move their big bodies quickly should a predator attack.

But as soon as they enter REM sleep their muscles lose the ability to hold them up and they have to lie down, making them more vulnerable.

Flexible sleep schedules

Whether it's shift work or taking care of young children, some of us get little sleep in the night and have to sleep at other times.

Northern elephant seals face this dilemma when they are out for months at sea: How do they maximise their foraging time and minimise their vulnerability to predators?

Unlike dolphins they can't sleep with half their brain awake and one eye open. So what do they do?

A study that strapped sleep monitors to elephant seals revealed that when they're in the ocean they limit their sleep to an average total of just two hours a day. The only other mammal that sleeps this little is the African elephant.

When the elephant seals first nod off into slow wave sleep, they free fall towards the ocean floor.

But once they enter REM sleep they flip upside down and fall, spinning in a circle, suggesting that like us, they are paralysed during this sleep phase. Sometimes they even hit the sea floor and continue sleeping.

Just as well they only sleep in short spurts and some days have no sleep at all when at sea.

Once they're back on land they flake out for around 10 hours a day.

Extreme sleep fragmentation

Some of us complain about broken sleep when we wake up in the night and we might rely on naps to get us through the day. 

The truth is there's no reason why we can't get by having our daily sleep in instalments, as long as some of these are long enough to give us the deep sleep we need.

But imagine surviving on microsleeps alone?

Researchers have found that Antarctic chinstrap penguins get their sleep during nesting season by having 10,000 microsleeps a day, each nap lasting just four seconds.

"This is an extreme level of sleep fragmentation," Professor Lesku says.

"It's a thing in birds," he adds. They are always on the lookout: even budgies and domesticated pigeons rely on microsleeps, albeit only a mere 1000 a day.

When sleep-deprivation is an advantage

If humans miss out on just one night of sleep we can feel off, but some birds can go for weeks without it.

Professor Lesku studied male pectoral sandpipers on the Alaskan tundra during mating season and found that some deprived themselves of sleep for 19 days. And this paid off.

"There was huge variation in how much sleep the males were getting, but those males that were capable of enduring sustained wakefulness, those were the ones that were siring the chicks."

Professor Lesku thinks being able to stay awake for so long might send a message to females from the male that he has more energy and persistence, and is a more attractive mate.

Although he isn't sure how the males can endure such sleep deprivation, he believes it's a feat to be admired.

"I would challenge any human male to stay awake for three weeks and convince any woman in a bar that they should mate with him. I think they'll struggle."

Sleepy wombats and koalas?

OK hands up who likes to sleep all day?

While giraffes and elephants are short sleepers, koalas and wombats have a reputation for sleeping long. Really long.

Wombats are reported to sleep for 18 hours a day and koalas even longer.

But Professor Lesku is dubious about the claims.

"We've never recorded brain activity from either of them."

He says these figures are based on observing their behaviour and assuming they must be asleep because they look asleep to us.

When you approach a koala in the wild that is wide-eyed and looking around, as soon as it clocks you it closes its eyes, lowers its head and freezes, he says.

"I think it's just sheltering because it wants to blend in."

Professor Lesku says studies in sloths support this conclusion. In the lab they sleep 16 hours a day, but it's a different story in their natural habitat.

"When folks went and studied what their sleep looks like in the wild … they were just sleeping nine hours a day and the rest of the time [they were] awake but not moving."

Octopus dreaming

For a long time it was believed that we were among a select group of animals that have two distinct states of slumber, including REM sleep, when we have our most vivid dreams.

But a growing number of creatures, including fruit flies, cuttlefish, a bearded dragon and Argentine tegu  challenge the assumption only birds and mammals have REM sleep.

Scientists recently measured the brainwaves of an octopus called Octopus laqueus and discovered it also had a sleep state that resembles human REM sleep.

During its REM-like sleep the octopus twitches as it fires off 'chromatophores' that cause rapid colour changes in its skin.

Colour changes usually plays a role in camouflage and communication but what's it doing while the animal is asleep?

Scientists think the octopus could be practising colour changes or re-playing hunting memories: re-living and learning from its waking experiences similar to dreaming.

Snoring for months on end

Generally speaking, it's a good idea for animals to sleep quietly so predators don't notice them when they're in the wild, says Professor Lesku.

But some animals do snore: and not just short-nosed dogs whose breeding has given them breathing problems.

Hibernating black bears are another animal that make such charming noises.

For six to seven months over winter they slow their bodies down to 25 per cent of their normal metabolism, which means they are technically in a different state to normal sleep.

During this time they only move every few days and don't eat, drink, or go to the toilet.

But they do snore!