“EIGHT HOURS labour, eight hours recreation, eight hours rest,” said Robert Owen, a Welsh textile-maker, in 1817. His aim was a fairer work week, but the idea that sleeping eight hours a night is best has since become the received wisdom. Is it right?
The quest for the optimal amount of shut-eye
Not getting enough kip can certainly be a problem. A bad night’s sleep can lead to a drop in cognitive abilities, increased stress and general grumpiness the following day. Chronic sleep deprivation is associated with cognitive decline, psychiatric disorders and even an early death. But too much sleep may also be bad for you. The evidence suggests a U-shaped relationship between health and sleep duration, with both insufficient and excessive slumber being worse than an optimal middle range.
In 2015 two big studies tried to work out exactly where that range lies. The first, published in Sleep Health, assessed 575 studies published between 2004 and 2014. For those aged 18-64, the recommended amount was between seven and nine hours a night, but that fell to just seven or eight hours for those over 65. The second, published in Sleep, analysed 311 studies. It concluded that around seven hours of shut-eye was the best target, but noted uncertainty about the negative effects of regularly sleeping longer than nine hours.
A more recent study, published in Nature on May 13th, refined these estimates by studying the effects of sleep duration on specific parts of the body. Its authors used the idea of “biological clocks”, which try to work out whether a person’s physiology is in better or worse nick than their chronological age would suggest. The researchers applied the concept to individual organs. Someone with a brain pathology, for example, might have a “brain age” that is higher than their actual age, explains Junhao Wen, a computational neuroscientist at Columbia University and self-admitted light sleeper, who led the study. Someone sporty might have muscles that look younger than their years.
Dr Wen and his team tracked the relationship between sleep duration and 23 different biological clocks across 500,000 adults (much of the data came from the UK BioBank, a big database). They found the U-shaped pattern across nine body systems, including the brain, lungs, liver and skin. Their findings suggest that the optimal amount of sleep is 6.5-7.8 hours for women and 6.4-7.7 hours for men.
Those struggling to get a full night’s rest should not despair. Applying population-level results to individual lives is tricky, for one thing. For another, disentangling cause and effect is not easy. Are some organs prematurely aged because their owners sleep too little? Or is some other factor causing both the ageing and the lack of sleep? Besides, says Michael Grandner, a psychiatrist at the University of Arizona, duration is just one measure. Quality, regularity and continuity of sleep matter, too. Dr Garner’s advice is: “Don’t freak out about any particular number, unless it’s a very unusual number.”
That looks like sage advice. One irony of sleep is that worrying about it can make things worse. A 2025 survey from the American Academy of Sleep Medicine found that 76% of respondents had lost sleep due to worries about sleep. Dr Wen advises listening to what your body is telling you, and getting enough that you feel refreshed when you wake up. Aim for six to eight hours a night, in other words, but don’t toss and turn over the precise number.
