How much do we really know about sleep? There is no single activity that humans do more: if you live to be 90, you will probably spend 32 years asleep. It is as vital for us as eating or drinking water. Sleep deprivation will kill you as surely as starvation. It is an activity we share with every other animal species, from cockroaches to chimpanzees. Yet we do not fully understand why we do it. Sleep scientists are locked in furious disagreements about what it’s for. Some suggest it’s to do with memory; some suggest it’s about clearing toxins from the brain; others suggest a combination of several factors. Others are even trying to establish whether humans can do without it altogether, using pharmaceutical drugs. Whatever is going on when we sleep, we do know one thing: we’re not doing it enough, and we’re doing it badly. We’re dosing ourselves with caffeine and nicotine to stay awake, and knocking ourselves out with alcohol and sleeping tablets, and all the while stopping ourselves from getting the healthy, normal sleep that lets our brains function. “The question of why we sleep is incredibly interesting,” says Prof Russell Foster, a neuroscientist and sleep specialist at the University of Oxford. “You have to disentangle it into two parts. One, what’s going on in the human brain as we sleep, and two, why did sleep evolve? They’re related, but they’re not the same.” From an evolutionary point of view, on the surface, sleep is baffling. Every member of the animal kingdom is forced to spend perhaps a third of its life unconscious and vulnerable: unable to feed, unable to watch for predators, unable to protect its offspring. “Everything sleeps. Not just humans, not even just vertebrates. Most of our studies of, for example, the genetics of sleep have been done in fruit flies,” says Adrian Williams, a professor of sleep medicine at King’s College London. Any animal that could do without it, you might think, would be at a huge advantage. And yet none does. “If sleep doesn’t serve some vital function, it is the biggest mistake evolution ever made,” as the great sleep scientist Allan Rechtschaffen said.
Christopher Nolan's Inception, above, was inspired by lucid dreaming But perhaps it’s not as mysterious as we think, and perhaps it does not have just one vital function, but many. The Earth has been spinning around the Sun for five billion years. Life, in some form or another, has been clinging to its surface for at least 3.5 billion of those. And for all of that time, it has had to deal with the abrupt and dramatic changes in light and heat that come with day and night. “Early on in the cycle, animals, and indeed plants, became adapted to the varying pressures of the light-dark cycle,” says Foster. They developed what we now know as a biological clock: a fine-tuning of physiology and behaviour to deal appropriately with that cycle. “Animals have made an evolutionary commitment to being active at one stage or another of the cycle,” says Foster. “So, if you’re a nocturnal animal, you’ve got big ears and eyes, and maybe big whiskers. That’s pretty much useless during the day, and an animal adapted to the day is useless at night.” It’s good practice, then, to hide yourself away during the hours of (in our case) darkness. And if you’re going to be out of action for a predictable period of time every day, says Foster, you might as well use that time for something. “Cells and bodies have to do a whole raft of housekeeping functions,” he says – clearing out toxins, bringing in new fuel – and brains, especially, have a lot to deal with. “All this information is streaming in during the day: it needs, at some stage, to be processed and packaged.” A recent paper found that one activity of sleep is clearing a toxin called beta-amyloid protein, which is associated with Alzheimer’s disease, out of the brain. Not all sleep scientists would agree with him, but Foster thinks that the correct evolutionary answer to the question “why do...
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