Why We Sleep

February 24, 2017

They say that an elephant never forgets. It is also often stated that one of the functions of sleep is to consolidate memories. If both of those things were true, then you’d expect elephants to sleep a lot – but the truth is, the massive pachyderms, which have the biggest brains of any land mammal, sleep just two hours each night.

Even though we sleep almost every night of our lives, it is also one of the most commonly misunderstood aspects of behaviour. It turns out that a lot of common ideas about sleep, much like the example above, are simply incorrect.

Have you heard, for example, that thanks to electric lighting and the faint glow emanating from the smartphone screens we stare at before going to bed, we get fewer hours of shut-eye than our hunter-gatherer ancestors?

Many people have heard this so many times through the media that they believe it,” says Jerry Siegel, director of the University of California Los Angeles Center for Sleep Research. He admits it’s a compelling story, even though it is – probably – completely untrue. “The trouble is we don’t really have any data on this,” he says. “The devices that we use to measure sleep weren’t invented until long after the invention of electric light.”

Since it’s impossible to figure out how much time our ancestors spent sleeping, Siegel decided to do the next best thing. He travelled to Tanzania, Namibia, and Bolivia, spending time with contemporary hunter-gatherer groups. These people are born into an environment that is as close as can be found today to the one our ancestors would have lived in.

For their entire lives, these hunter-gatherer societies have lived – and slept – without any of the modern devices we suspect disturb our own rest. Several thousand miles separate the two groups in Africa, while the third is descended from a group that migrated out of Africa, travelled through Asia, crossed the Alaskan land bridge before then moving through North America and into South America. Despite this remarkable divergence, all three groups sleep about the same amount of time each night: six-and-a-half hours, on average. According to Siegel, there’s no reason to believe our ancestors would have slept any more than that.

For most humans – living in modern societies with all the trappings of technology and electricity – the amount of time they spend kipping is between six and eight hours a night. So not only did our ancestors not sleep longer than we do, but they may have got slightly less sleep than some of us.

We also generally sleep in the comfort of our climate-controlled homes, on comfy mattresses with fluffy pillows, where our biggest worry is who is hogging the covers and whether to allow Fido to sleep on the bed too. Our human ancestors slept instead on rocks, dirt or possibly on tree branches, without the creature comforts of down comforters or central heating. They could not use blackout blinds to let them lie in long after the sun came up, nor could they hope to avoid weather or insects. They also had to worry about being picked off by the occasional predator or attacked by a rival group while they slept. It’s no wonder they likely got little more than six hours each night.

Yet there is another myth about how our ancestors slept – that they napped in several short chunks through the night, rather than in one long slumber. On that too, according to Siegel, we’re wrong. This erroneous assumption, he blames on our pets.

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