In making their recommendations, the experts took into account the health benefits, but also the risks, associated with sleep. Too little sleep over several nights leaves you tired, unable to concentrate, depressed, anxious and, eventually, if it continues, at an increased risk of diabetes, high blood pressure and obesity. Too much sleep is associated with much the same problems.
So how much is the right amount of sleep? The new guidelines not only give recommended amounts, but also state what might be appropriate for different ages. Children aged six to nine need nine to 11 hours a night, but may get by on seven to eight. Teenagers need eight to 10 hours. Seven hours may be OK for some, but sleeping more than 11 hours a day may be detrimental to their health, although some may need that much during puberty.
Dr Lydia DonCarlos from Loyola University, Chicago, one of the experts on the study, says that the circadian rhythm of teenagers naturally shifts to make them feel sleepy later at night and to wake up later. This is a normal phenomenon and nothing to do with being addicted to social media. She warns that teenagers should still try to get enough sleep on a daily basis, rather than building up a sleep debt to pay off at weekends. “You can never quite make it up,” she says.
Adults aged 18 to 64 need to sleep for seven to nine hours a night, but some cope on six. For people over the age of 65, the recommended amount is between seven and eight hours, although some survive on five hours sleep (often waking up earlier and napping during the day).
These recommendations are based on a thorough analysis of the studies. The methodologies do vary – some are based on how much sleep people reported they had had (which tends to include time spent in bed) and others are based on research carried out in laboratory conditions.
The experts did not look at quality of sleep (for example, whether people woke up in the night and couldn’t get back to sleep) or its architecture (how much was rapid-eye movement and how much was slow-wave sleep). Some people may survive on less than recommended amounts because they get higher-quality sleep.
DonCarlos says that more research into sleep is needed. “We spend one third of our life asleep, but we know so little about it.” At least knowing how much to aim for is a good start.