How cool are sloths? It’s time to celebrate! On this International Sloth Day, let’s celebrate sleep! Sloths in the wild sleep around 10 hours a day. Two hours is enough for a fruit fly, but cats sleep need 15 hours a day, cows sleep just four hours, and humans?

Well…how long do you need to sleep?

Stages of sleep

When you’re asleep, your brain produces characteristic brain waves. We graph these brain waves by measuring electrical activity inside your head.

We know that humans sleep in repeating cycles, each around 90 minutes long. Each sleep cycle includes different types of sleep:

• Transition: You are drifting between being asleep and awake. You can be easily woken.
• Slow-wave sleep (SWS or non-REM): You are deeply asleep. Your eyes don’t move. Your body temperature drops.
• Rapid Eye Movement (REM) sleep: You’re still asleep, but your eyes are darting around, your heart is pumping fast, your brain is working hard…

If you sleep for ten hours a night, you’ll spend around two hours in REM sleep. We think REM sleep helps you learn and create memories. Most dreams happen during this time.

Dragon dreams

Gilles Laurent studied the brain waves of Australian bearded dragons. He found their sleep also cycled through REM and non-REM sleep, but much faster than humans: one cycle every 80 seconds, around 350 times a night.

This shared pattern of REM and non-REM sleep means the way we sleep and form memories may have evolved more than 300 million years’ ago.

“It suggests that these features existed also in dinosaurs, which are the reptilian ancestors of birds,” Gilles says.

Does this mean lizards dream?

“It depends a lot on how one defines dreaming,” says Gilles. “If one defines dreaming simply as off-line replay of previous activity…then I’d venture to say that pet lizards do dream.”

Animalzzzz

All animals need to sleep, but there are different ways to catch zeds.

Some animals—like dolphins and ducks—sleep with only half their brain, so the other half can stay alert and awake.

Many large herbivores—like elephants, cows and horses—can sleep standing up, but must lie down for REM sleep.

Newborn kittens and puppies only have REM sleep, suggesting this type of sleep is important to early brain development. As they get older, they have less REM sleep.

The platypus enjoys more REM sleep than any other mammal. Platypus have been seen ‘swimming’ or munching imaginary food while they’re asleep.

Even insects sleep. Fire ant workers nap for about a minute, 250 times a day; their fire ant queens sleep for six minutes, 90 times a day.

Sleep your way to success

Getting enough sleep is linked to improved creativity, concentration and memory, even in animals. Tired fruit flies, for example, make more mistakes and struggle to remember important things.

Sleeping can also help animals make healthier choices. Mice who don’t get enough REM sleep are more likely to eat fatty and sugary foods.

Are you human? Aged 6–12 years? Aim for 9–12 hours of sleep every day.

Help! My hamster’s dead!

Limp and floppy? Don’t panic. Your beloved pet may simply be hibernating.

Hibernating animals seem to ‘sleep’ away the winter, but we can tell from their brain waves that they’re still awake. When an animal ‘wakes’ from hibernation, it hasn’t actually slept, so it needs lots of proper naps to recover. Squirrels, frogs, mice, bats and even hamsters hibernate.

This article originally appeared in CSIRO’s Double Helix magazine.