Why Sleep Might Be the Most Underrated Part of Your Health

Ask people what they do for their health and you will hear about walking, vegetables, perhaps a gym membership. Sleep rarely makes the list. It is what happens when the day runs out, and the first thing we trim when life gets busy.

That attitude has a long history. In 1999 a team at the University of Chicago noted, in The Lancet, that sleep debt was widely believed to have no adverse effect on health. Their own experiment suggested otherwise, and a quarter of a century of research since has turned sleep from an afterthought into one of the pillars of health. It even earned a formal place on the American Heart Association's checklist for a healthy heart.

A clean-up crew, or something subtler?

The most arresting idea in modern sleep science came from Maiken Nedergaard's laboratory at the University of Rochester. In 2013, in a paper in Science, her team reported that in sleeping mice, the spaces between brain cells expanded by about 60 per cent. That allowed cerebrospinal fluid to flow more freely through brain tissue and, they found, sped up the removal of waste products, including beta-amyloid, the protein that clumps in the brains of people with Alzheimer's disease.

The researchers called this plumbing the glymphatic system, a nod to the glial cells that help run it and to the lymphatic system that drains the rest of the body. The picture of sleep as a nightly rinse cycle for the brain was irresistible, and it quickly entered popular culture.

Science, however, rarely stays tidy. In 2024 a team at Imperial College London, led by Nicholas Franks and William Wisden, published a study in Nature Neuroscience that reached the opposite conclusion. Injecting a fluorescent dye into the brains of mice, they found clearance was about 30 per cent lower during sleep than during wakefulness, and about 50 per cent lower under anaesthesia. The team said they were surprised, and acknowledged that different molecules may be cleared in different ways.

So the jury is out on exactly how sleep affects the brain's housekeeping, and nearly all of this work has been done in mice. What is not in doubt is that something important happens. In the Whitehall II study of British civil servants, led by Séverine Sabia at UCL and Inserm, people who reported sleeping six hours or less in their fifties and sixties were more likely to develop dementia over the following decades than those who slept around seven hours. The authors were careful to say this does not prove that short sleep causes dementia. But it fits a growing body of evidence linking the night to the long-term health of the mind.

What a short night does to the body

Sleep is not only about the brain. The 1999 Chicago study, led by Karine Spiegel and Eve Van Cauter, restricted 11 healthy young men to four hours in bed for six nights, then let them recover with 12 hours a night. During the sleep debt, their glucose tolerance fell, evening levels of the stress hormone cortisol rose and the sympathetic nervous system, the body's accelerator, became more active. The authors noted that these changes resembled those of normal ageing.

It was a small laboratory study, but large population studies point the same way. A 2010 meta-analysis led by Francesco Cappuccio at the University of Warwick pooled data on more than 1.3 million people. Short sleepers had about a 12 per cent higher risk of dying during follow-up than people sleeping a typical seven to eight hours. Long sleepers had about a 30 per cent higher risk, though Cappuccio suggested that very long sleep is often a sign of existing illness rather than a cause of it.

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We treat sleep as what happens when the day runs out. The body treats it as essential maintenance.

That accumulating evidence is why, in 2022, the American Heart Association added sleep duration to its list of the core measures of cardiovascular health, renaming it Life's Essential 8. Alongside blood pressure, cholesterol, blood sugar, weight, diet, activity and not smoking now sits a recommendation that adults average seven to nine hours a night.

The underrated part of the underrated part

Duration gets the headlines, but timing may matter just as much. In 2024 a team led by Daniel Windred and Andrew Phillips at Flinders University in Adelaide, writing in the journal Sleep, analysed wrist-sensor data from more than 60,000 UK Biobank participants. They calculated a Sleep Regularity Index, a measure of how similar each day's pattern of sleep and wake was to the next.

People with more regular sleep had a 20 to 48 per cent lower risk of death over the following years than those with the least regular patterns, after adjusting for many other factors. Strikingly, regularity predicted mortality better than sleep duration did.

The likely reason is the body clock. Nearly every organ keeps its own daily rhythm, coordinated by a master clock in the brain. Erratic sleep keeps resetting it, a little like taking a small flight across time zones several times a week.

Giving sleep its due

The NHS says most adults need between seven and nine hours, and its advice for getting there is refreshingly unglamorous. Go to bed and get up at roughly the same times, including on days off. Wind down in the hour before bed with reading, breathing exercises or calm music rather than a screen. Keep the bedroom dark, quiet and cool. Avoid large meals late in the evening, keep caffeine in check and stay away from alcohol and nicotine in the hour or two before bed. If you are lying awake after about 20 minutes, get up and do something relaxing until you feel sleepy.

Sleep naturally changes with age, becoming lighter and more fragmented for many people. But persistent insomnia is not something to simply put up with. If changing your habits has not helped, or if you have struggled to sleep for months, the NHS advises seeing a GP. Talking therapies such as cognitive behavioural therapy are often recommended, and sleeping pills are prescribed only rarely and for short periods. If your breathing stops and starts in the night, or you gasp or snort in your sleep and feel exhausted by day, tell your doctor, as these can be signs of sleep apnoea.

None of this is as exciting as a new supplement or a smart ring. But for a habit we spend roughly a third of our lives doing, it seems worth getting right.

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This article is for general information and is not a substitute for personal medical advice. If you have concerns about your health, please speak to your GP or a qualified professional.