For humans, sleep is a necessity, a mystery, and a luxury. It’s not known why we need it but we do—and an hour more or less of it can make your day either great or grumpy.
Most animals sleep, too, says Jerome Siegel, a psychiatrist at the University of California, Los Angeles, but in ways that are just about as varied as the animal kingdom itself. These variations include duration and depth of sleep, and even how it works in the brain.
From dogs that doze off and on all day to dolphins that sleep using only half their brains, here’s a look at the many different ways that animals sleep.
Humans, like all other great apes, are monophasic sleepers, meaning we sleep in one long interval during a 24-hour period. Bonobos, chimpanzees, gorillas, and orangutans all also build sleeping platforms in trees, away from predators and insects, a jungle version of a bed. Gorillas sleep for 12 hours but orangutans get around the same eight hours that humans do.
In some other primates, as in most mammals, sleep is polyphasic, with several alternating periods of sleep and activity in a 24-hour cycle. Dogs have wake-sleep cycles of about 83 minutes and get a little more than 10 and a half hours of sleep per 24-hour cycle.
The reason great apes have such long, luxurious sleep compared to the fitful, shorter sleeps of their monkey cousins has to do with those sleeping platforms. Monkeys have to balance on hard branches where they are easily awakened by potential danger or other monkeys—which is helpful to them but not good for extended sleep.
When apes started getting bigger, the branches they once slept on could no longer hold their weight—so they started building something that would. Being able to lay down, away from the dangers of predators and other distractions allowed them to sleep longer, more securely, and more deeply. A 2015 study showed that orangutans do, indeed, sleep better than their baboon cousins. The apes’ cognitive abilities, the study says, may improve on the day following that longer, deeper sleep.
Sleeping with half a brain
Dolphins, meanwhile, can stay alert with half of their brain while the other half can fall into a deep sleep. This enables dolphins to sleep with one eye open, looking for predators.
“Dolphins are basically alert 24 hours a day for their entire lives,” Siegal says.
This sleeping pattern—which dolphins share with other cetaceans, manatees, eared seals and some birds—is called unihemispheric slow wave sleep, a deep state of sleep in which rapid eye movement or REM sleep does not occur.
REM sleep is the sleep state in which the brain is most active, breathing becomes more rapid, and most muscles become temporarily paralyzed. The importance of REM sleep has been a subject of scientific debate concerning how much of a role it plays in memory and learning. Dolphins are highly intelligent but possibly never experience REM sleep, says David Raizen, a neurologist at the University of Pennsylvania,…