The surprising truth about why we sleep and how much we need
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.
"I think the origin of that [idea] is people have cats and dogs, and that's what cats and dogs do – they do sleep that way," he says. "But primates don't." We are but the latest in a long line of species that tends to snooze in one long, uninterrupted block of sleep each night. That's not to say that apes and monkeys don’t take the occasional mid-day nap, or that they don't wake up in the middle of the night from time to time. But, just like in our own species, that's not quite the norm.
Indeed, Siegel's cross-cultural study found that modern hunter-gatherer groups almost never napped in the winter, and only napped slightly more often during the summer, presumably as a means of escaping the worst heat of the day. And even then, he says, the average person only took a daytime nap every fifth day or so.
But there is one tiny way in which the myth holds up. The people Siegel studied all lived fairly close to the equator. As you move to higher latitudes, the night can last up to 16 hours in the winter, so living in that kind of environment may have led Northern European ancestors to fragment their evening slumber during that part of the year. But because we have cleaved our sleep patterns from the natural cycles of the seasons, even in Northern Europe most modern-day humans sleep through the night, perhaps just waking up for a quick visit to the bathroom.
Having settled two of the most pervasive myths regarding sleep behaviour, Siegel has now turned to other, more fundamental questions about the nature of sleep. Why do we even do it?
If it played a role in memory consolidation or some other brain function, then you wouldn’t expect the big brown bat to get a whopping 20 hours a day, while the much larger and cognitively complex African elephant survives comfortably on just two.
Instead, Siegel wonders whether sleep may not be a biological requirement itself, but rather evolution's way of maximising productivity. As he wrote in Nature Reviews Neuroscience in 2009, perhaps sleep provides a means of "increas[ing] the efficiency of behavior by regulating its timing and by reducing energy use when activity is not beneficial."
西格尔认为，睡眠本身可能并非一种生物学需求，而是在进化过程中产生的、实现生产力最大化的一种行为。他在2009年发表于《神经科学自然评论》（Nature Reviews Neuroscience）杂志上的一篇论文中写道，睡眠的作用可能是"在生物活动不再有益的时间段，通过优化时间使用及降低能量用量而提高行为效率"。
It's a common trick in both the animal and plant kingdoms. Some trees shed their leaves in the autumn and cease photosynthesising, which could be thought as a kind of botanical slumber. Bears hibernate in the winter, in part to avoid fruitlessly expending energy hunting and foraging at a time when there is not much food to be found.
Other mammals, such as echidnas, enter a sleepy-state known as torpor, where their metabolism slows down to barely a whisper to help them get through hard times. Perhaps sleep is simply our version of such "adaptive inactivity", allowing us to be productive during the daylight hours while avoiding overexertion - and, historically, exposure to predators - at night, while still permitting us to awaken easily if necessary.
Or, to put it another way, maybe it's selective laziness.