WE ALL know 8 hours is the magic number for a decent night’s sleep. Or is it?
Nobody seems to know where this number came from. In questionnaires, people tend to say they sleep for between 7 and 9 hours a night, which might explain why 8 hours has become a rule of thumb. But people also tend to overestimate how long they have been out for the count.
According to , who studies sleep at the University of California, Los Angeles, the 8 hour rule has no basis in our evolutionary past – his study of tribal cultures with no access to electricity found that . “And those people are pretty healthy,” adds at the University of Surrey, UK.
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Sleep – a user’s guide: We answer all the questions keeping you up at night
So perhaps 8 hours is the wrong target and we can get by just fine with 7. This seems to be a minimum requirement. A recent analysis in the US concluded that regularly getting less sleep than that increases the risk of obesity, heart disease, depression, and early death, and recommended that .
By this benchmark, recent reports seem to suggest we are walking around in a state of sleep deprivation. The US Centers for Disease Control and Prevention estimates that 35 per cent of US adults are getting , and a survey in the UK found that . The media widely state that we are getting less sleep than we used to. The implication is that it’s taking a severe toll on our health.
Not everyone is convinced. “Sleep has not changed in the past 100 or so years,” says sleep scientist , who takes the idea to task in his new book . That’s a notion backed up by a recent review of scientific literature on sleep between 1960 and 2013, which found no significant link between sleep duration and the year a study was conducted.
What studies have shown is that the amount of sleep we need is and varies among individuals. Exactly which genes are involved is not well understood, but a recent study of over 50,000 people found one gene variant that . The amount of sleep you need also changes as you age. Taking this into account, the US National Sleep Foundation updated their guidelines last year, and came up with a recommended range of 7 to 9 hours for adults, but with added leeway of an hour either side to account for natural variation (see chart).
So how much is enough for you? A general rule of thumb is that you shouldn’t need an alarm clock to wake you up in the morning. Dijk also recommends jotting down what time you switch off to keep an accurate record of how much sleep you get.
While giving yourself more time under the covers can be good for your health, says of Arizona State University in Tempe, take care – you can have too much of a good thing. “There seems to be a sweet spot for all health-related behaviours. For sleep that seems to be about 7 hours.” Regularly getting 8 hours or more could send you to an early grave, he says. “Typically, the association is at least as strong, often stronger, than the association of short sleep with mortality.”
Just why this is remains a mystery. It could come down to the simple fact that when we are asleep we are moving very little, and there’s plenty of evidence to show that inactivity is bad for you.
And although this might not matter if you are active during the day, it could be that people who spend more time asleep do less exercise, possibly because they simply have less time, Youngstedt suggests. Long sleep is also associated with inflammation, an immune response linked to everything from depression to heart disease. And you might not need as much sleep as you think, says Youngstedt. “Many sleep for a long time out of habit or boredom, and we have found that they can tolerate mild sleep restriction,” he says. So try cutting down and see how you feel.
What of those grating individuals who claim to get by just fine on a few hours each night? They probably are sleep-deprived, but have got used to the effects and now fail to notice them as strongly. Or else they may simply be napping later on in the day. Only a tiny minority of us, probably less than 3 per cent, can get by on 4 to 6 hours of sleep with no problems at all. at the University of California, San Francisco, and her colleagues found a particular gene in a family of these natural short-sleepers. When the team engineered mice to express this short sleep gene, they and seemed to whizz through the non-REM stages of sleep faster than non-engineered mice.
The team thinks this gene variant interacts with proteins that are at the core of the circadian clock, opening up the tantalising possibility thatwe could one day genetically engineer our way to a shorter night’s sleep, without the downsides. In the meantime it is comforting to know that, for most of us, getting stuff done on very little sleep is, so far, physically impossible.
We answer all the questions keeping you up at night in “Sleep: A user’s guide”
This article appeared in print under the headline “How much shut-eye do I need?”
