For years, the mantra has been drilled into us: eight hours of sleep a night is the gold standard. But a major new study, drawing on data from around half a million people, suggests the true sweet spot for healthy ageing is far narrower — and may not include eight hours at all.
Researchers analysing the sleeping habits of participants in the UK Biobank — a long-running health project in Britain — found that those who slept between 6.4 and 7.8 hours a night appeared to age more slowly than those who got significantly more or less rest. The findings, published in Nature, showed this link across multiple bodily systems, including the brain, lungs, liver, immune system and even the skin.
What the study found
The UK Biobank is one of the most extensive health databases in the world. Between 2006 and 2010, 503,317 volunteers aged 40 to 69 were recruited from within a 25-mile radius of 22 centres across Scotland, England and Wales. Of these, 54% were female and 46% male; the majority (94.6%) were of white ethnicity. The study holds more than 10,000 variables per participant — including biological samples, physical measurements, brain and body imaging, activity tracking and lifestyle questionnaire data — and has produced over 9,000 peer-reviewed publications.
By comparing participants’ reported sleep habits with measures of biological ageing across the body, the research team identified a surprisingly narrow ideal range. Sleeping between 6.4 and 7.8 hours per night was associated with a slower rate of biological ageing — the rate at which cells and organs deteriorate — compared with both shorter and longer sleep durations. The effect was consistent across the brain, lungs, liver, immune system and skin.
The risks of too little and too much sleep
The study found that getting fewer than six hours of sleep a night was strongly linked to a higher risk of several health problems, including heart failure, type 2 diabetes, anxiety and depression. Broader research supports these findings: adults who sleep five hours or less have a 200–300% higher risk of coronary artery build-up, and short sleep is associated with a 45% increased risk of coronary heart disease. Consistently logging less than seven hours also increases the risk of atrial fibrillation, cardiometabolic syndrome and reduced overnight blood pressure dipping. Sleep deprivation can weaken the immune system, making individuals more susceptible to infections, and accelerates visible signs of skin ageing such as wrinkles, dullness and loss of elasticity.
At the opposite extreme, sleeping for more than eight hours was not linked with perfect health either. The study reported associations between longer sleep durations and conditions including major depressive disorder, bipolar disorder, schizophrenia, ADHD and alcohol dependency. Other research has found that sleeping more than nine hours per night is linked to an increased risk of cardiometabolic syndrome, stiff arteries, stroke and death from heart disease or stroke. Both short sleep (under seven hours) and long sleep (over nine hours) have been associated with at least a doubling in dementia risk, as well as silent brain injuries that can foreshadow stroke and dementia.
A slight gender difference emerged: women appeared to fare slightly better with around 6.48 hours of sleep, while the figure for men was 6.42 hours. The gap is small, but it matches previous findings that women may need marginally more rest than men.
Limitations to consider
Before anyone reaches for a stopwatch, it is important to understand the study’s limitations. The research relied on participants reporting their own sleep habits through questionnaires. Self-reported data can be unreliable — some people may misremember how long they slept or simply guess. This means the precise numbers should be treated with caution.
Sleep needs also vary considerably from person to person. Age, sex and existing health conditions all influence how much rest an individual requires. The UK Biobank participants, while large in number, are not fully representative of the general population: they were generally slightly wealthier and healthier than the national average at the time of recruitment, a phenomenon known as “healthy volunteer bias.” All were aged between 40 and 69 at enrolment, so the findings may not apply equally to younger or older age groups. And because 94.6% of participants are white, the results may not translate directly to other ethnic groups.
Despite these caveats, the study’s scale — half a million people and a vast array of health data — gives its conclusions considerable weight. The researchers used measures of biological ageing that go beyond chronological age, looking at how fast cells and organs were deteriorating, which strengthens the evidence that sleep duration is a modifiable factor in healthy ageing.
The case for sleep regularity
Interestingly, a separate study published in the journal Sleep suggests that how consistent your sleep is may matter even more than how many hours you log. Researchers using accelerometer data from more than 60,000 UK Biobank participants found that higher sleep regularity — keeping consistent sleeping and waking times — was associated with a 20–48% lower risk of all-cause mortality, a 16–39% lower risk of cancer mortality, and a 22–57% lower risk of cardiometabolic mortality. Sleep regularity was a stronger predictor of overall mortality risk than sleep duration, even after adjusting for other factors. It has also been linked to improved outcomes across alertness, cardiovascular and metabolic health, inflammation and mental health.
The message is clear: while many people fixate on hitting a specific target each night, the bigger challenge may be sticking to a routine. Your body clock appears to value reliability as much as it values rest.