Steven Poole reviews Why We Sleep by Matthew Walker

Are you getting enough sleep? If it’s routinely less than seven to eight hours a night, the answer is almost certainly no. Thanks to electric light, the blue glare of smartphones and tablets, and the pressures of modern life, people in modern Western societies are chronically sleep-deprived. And, as Matthew Walker, the neuroscientist sleep researcher, argues, this is much more of a problem than we all think. Driving while drowsy is more dangerous than driving while drunk. And not getting enough sleep compromises your immune system, doubling your risk of cancer and other serious illnesses.

Walker explains cheerily what we know about the body’s circadian rhythm, as well as the causes of jet lag, what we can healthily do to combat insomnia, and the different stages of sleep, which overall must fulfil some very important biological function because every single animal we know of does it. (It used to be thought that sharks didn’t sleep, but then we discovered that it just looks that way because they have no eyelids. Imagine.) The book’s hard sell, indeed, is that science is finally uncovering the secrets of sleep, but it turns out that there is a lot that is still unknown or controversial, including why exactly sleep has the form and structure (an asymmetric series of repeating cycles) it does, or why dreams are the way they are.

Where the book shines is in its descriptions of what the author’s own experimental results have shown. One function of sleep, for example, seems to be to shift “recently acquired memories to a more permanent, long-term storage location in the brain”; but it also prunes recent data and throws away what is not deemed useful. In these ways more space is made for new stuff. It also turns out that REM or dream sleep – which we get most of in the last two hours of an eight-hour night – is crucial for consolidating motor-skill improvements. REM sleep also seems to be the arena for “emotional processing”: indeed, Walker and his colleagues found that “a dream-starved brain cannot accurately decode facial expressions”. So if you sleep only six hours a night you won’t get better at a physical skill such as yoga or playing the piano, and you’ll get personal interactions wrong in embarrassing ways. All very strong reasons to stay in bed.

On such subjects, the book is genuinely fascinating enough to keep you from dozing off, so by its own lights it should definitely not be read late in the evening. One does have to be able to tolerate the periodic feeling that the author is talking down to his readers, with unnecessary exclamation marks and warm ’n’ fuzzy metaphors (eg that sleep provides “a consoling neurochemical bath that mollifies painful memories”), or silly exaggerations, eg that sleep “truly” is a “panacea”. (No it isn’t; we will all still die, no matter how much we snooze.) But Walker is always enthused, and fellow haters of alarm clocks will enjoy his description of them as a “depraved” – and physiologically dangerous – way to wrench ourselves from the land of Nod.