Like the animal kingdom itself, the world of animal sleep is both vast and incredibly varied. There are roughly 60,000 vertebrate species on Earth—and scientists have studied sleep in fewer than 100 of them. We know giraffes sleep about 30 minutes a day, in 5 minute naps, while brown bats log almost 20 hours of daily shut-eye. We know walruses can sleep floating on the surface of the sea, or submerged underwater.  We think seabirds may forgo sleep for their thousands-mile long journeys—or they may be able to sleep in-flight. But there’s a whole lot about animal sleep that remains a mystery.

Animals have played an essential role in unlocking the mysteries of human sleep. At the top of the list? The rat. Rats have sleep needs that closely resemble human beings own need for sleep, and they’ve been used in countless scientific studies as a means to glean insight and information about how human sleep works, and why we do it. (Amazingly, the answer to that last question is still pending. For all our scientific inquiry into sleep, scientists still haven’t determined exactly why we sleep.)

A recent sleep study of dogs revealed for the first time that our canine friends experience a type of sleeping brain-wave activity—known as “sleep spindles”—that is also present in human sleep. Sleep spindles occur during non-REM sleep. In humans (and in rats), and they’re linked to memory function and our ability to learn new things, as well as to overall intelligence. This recent research suggests that’s true for dogs, too. Scientists discovered the more sleep spindles dogs experienced, the better they learned new skills.

Humans and other animals share a basic need for sleep and rest in some form. You might not think your sleep routine has much in common with a lion or a field mouse, but at a fundamental level, all mammals share a similar sleep cycle. A key similarity? REM sleep. That’s a stage of sleep when our brains become highly active, in ways similar to when we’re awake. It’s during REM sleep that we typically experience our most vivid dreams. Scientists are still working to better understand the purpose of REM sleep, but evidence suggests that it’s important for the brain’s processing of emotions, and of new information into memory. (We can think of REM sleep as a kind of internal nightly download period, which helps clear the brain’s slate to start fresh the next day.)

Mammals of all types experience REM sleep. That includes whales, who scientists think may enter brief periods of REM that are different from the rest of their sleep, when these and other marine mammals sleep with only one half of their brains at a time.

The presence of REM sleep among mammals also raises the distinct possibility that these animals dream. Scientists don’t know enough about animal sleep to say for sure, but there’s evidence that animals from cats to rats all experience something akin to dreaming during sleep. Maybe they’re dreaming about chasing each other!

Written by Caitlin Reynolds

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