How much do we really know about sleep? 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 we sleep?" is: lots of reasons. To examine those reasons in more detail, we need to talk about Foster's second question: what's going on, physiologically, in our brains when we sleep? To answer that, in turn, we need to ask what we mean by sleep at all. We tend to think of sleep as just an absence of consciousness, because that's what we experience it as: we lie there for a bit, then we nod off, then we get up with a full bladder and wander to the lavatory, then we go back to sleep again until the alarm goes off. But there is more to it than that. For humans, there are four distinct phases of sleep: three largely dreamless phases of increasing depth, then a highly active period, known as "rapid eye movement" or REM sleep, when most of our dreams happen. "We pass from wakefulness through the sleep phases in a very orderly pattern," says Williams, going through all the phases about four or five times in a normal night. The first four hours or so you spend more time in "restorative" non-REM sleep; the second half, you spend more in dream-laden REM sleep. "Dreaming has a real biological function," says Williams. Rechtschaffen, one of the pioneers of sleep science, experimented on rats, forcing them to stay awake. They all died of weight loss, within two weeks, despite eating more than usual. But if you let them get some sleep, but just deprived them of REM sleep, they died within four. Francis Crick, one of the co-discoverers of the DNA double helix, was one of the first to suggest what's going on in REM and non-REM sleep. "His idea was that non-REM sleep is for repairing the body and REM sleep is for rewiring the brain," Williams says. The brain is very active during REM sleep; it uses as much oxygen as it does while it's awake. REM sleep is when we do most of our dreaming. We are fascinated by dreaming. The fascination comes, in part, because we feel there is something semi-mystical about dreaming. There are famous stories of people having great creative breakthroughs in their dreams: Paul McCartney claims that he woke up one morning with the tune for Yesterday already playing in his head; Dmitri Mendeleev is said to have seen the periodic table of elements in a dream, and when he wrote it down in the morning, "only in one place did a correction seem necessary." In a perhaps less epoch-making moment, Stephenie Meyer says she came up with the idea for her Twilight novels in a dream, as well. Experiments have shown that humans pass through four distinct phases of sleep Most of us won't come up with a scientific breakthrough or a bestselling vampire series in our dreams. But dreaming-time is still important. Foster believes the brain is making sense of events, that it is putting together a "jigsaw" of information and that the pieces that it can't fit into its usual model of how the world works create the bizarre associations of dreams - the "I was in the office, but it was also Ikea, and Tom Selleck was there" stuff. Those bits aren't kept for long-term memory, although we'll remember them if we wake in the middle of one. As Foster admits, the role of REM sleep in memory is unclear, as some studies have cast doubt on whether it has an effect on our recall. Also controversial is the phenomenon of "lucid dreaming": wakefulness in dreaming, when dreamers are aware of the dream and able to control their actions. Some doubt its reality - the philosopher Norman Malcolm pointed out that there is something absurd about saying "I dreamt that I realised I was dreaming, dreamt that I was affecting the course of my dream, and then dreamt that I woke myself up by telling myself to wake up." But, says Foster, it is a real phenomenon, and what's more, some people can train themselves to do it, much like the sci-fi film Inception. The internet is full of how-to guides, telling would-be lucid dreamers to ask themselves constantly during the day, "Am I dreaming?" so the habit carries on into sleep. Foster says the practice is "akin to substance abuse" in that you "trick the brain into behaving in a way that is slightly disconnected from the real world." As the rat studies showed, sleeping is essential for survival. But we struggle to get enough. While, on average, we need 8.1 hours" sleep a night, a study in 2006 found that the average American got barely six hours (although 7.5 were spent actually in bed). "We don't have as good data in the UK," says Foster, "but all the data we do have suggests that we are getting significantly less sleep today compared to the Fifties and Sixties." That 8.1 hours is only an average, of course: the actual amount of sleep you need will almost certainly be between six and 10. "It's taken from a very old study," says Williams, "in which they took a group of healthy young people on holiday and let them sleep as long as they wanted. The first couple of nights they slept for nine hours, and after that it settled. You'll notice the same, if you're sleep-deprived: that you reach the weekend and you have to sleep for longer, to catch up on sleep debt. This is called the "homeostatic drive," and every creature has it. "If you disturb a fish or a fruit fly or whatever during their rest period, it will need more sleep the following night," says Foster. And if it doesn't get it, the organism will seek more of it. Like starving something of food. In future, it may be possible to use drugs to reduce our sleep need, or even mimic it and thus do away with it altogether. Modafinil, a "eugeroic" or wakefulness-promoting drug that can keep people awake for days at a time, appears to do so without building up a sleep debt. Foster has said before that, in the relatively near future, it may be possible to reduce sleep need to a couple of hours a night, extending our conscious lives by nearly 50 per cent. For the time being, though, that is not a realistic option, and instead people are self-medicating with stimulants and sedatives. That would be bad enough, but, says Foster, the practice is spreading to a far more vulnerable group: schoolchildren. "Teenagers who get up late aren't lazy: they naturally need more sleep than adults," he says. The difference between a fiftysomething and a late teen is about two hours, so making a teenager get up at 7am is like making a 55-year-old get up at 5am. A study in America found that teenagers need an average of nine hours" sleep, but that a quarter were getting less than six-and-a-half. Foster says the situation is even worse in Britain. "I visited a school in Liverpool and spoke to a 13-year-old girl," he says. She said she sleeps fine, because she takes lots of sleeping tablets. I asked how she manages the next day, and she said she has about three Red Bulls before lunchtime. This is a developing, plastic brain, which is being insulted with these sedatives and stimulants, and we have no idea what the consequences of that are. What's strange about all this is how sleep, a third of our lives, takes up so little of our thinking. Medical schools dedicate almost no teaching to it; our education system worries about so much else, but not about the very simple and obvious fact that sleep deprivation ruins children's ability to learn. "This school in Liverpool was amazing," says Foster. The teachers were fantastic, it had all the books and equipment, but the kids were falling asleep in class, because they're not getting guidance on how important sleep is. No one's telling them, absolutely no one's telling them. We're educating our kids in so many areas - sex, hygiene, drugs - but nobody gives us any guidance about the single most important behaviour we experience. It's bordering on the criminal, frankly.