Revealed: How many hours of sleep you really need (spoiler: it's not eight)

New research from the University of Cambridge and Shanghai's Fudan University looked at the sleep habits of almost 500,000 adults between the ages of 38 and 73. Photo / Lux Graves, Unsplash

From smartphones that monitor REM to ever-changing "sleep hygiene" rules, we all know a good night's sleep is essential to wellbeing. Advice varies, however, on how long actually constitutes that restorative rest. British Prime Minister Margaret Thatcher famously got by on four hours, while the accepted standard is eight. I prefer nine – but according to a major new study, we should all be aiming for exactly seven hours' sleep, from our thirties to old age.

The research, from the University of Cambridge and Shanghai's Fudan University, looked at the sleep habits of almost 500,000 adults between the ages of 38 and 73.

Participants were asked about their sleeping patterns, mental health and wellbeing, and took part in a series of cognitive tests. Brain-imaging and genetic data were also available for almost 40,000 of the study participants.

Researchers discovered that both too much and not enough sleep could cause mental health problems and "worse cognitive performance".

The study also found that consistency mattered – so it's no good getting nine hours one night then six the next.

The researchers speculate that one reason for the "brain fog" caused by lack of sleep is that slow-wave deep sleep is disrupted, which may also stop the brain from clearing out toxins. They also found that the brain's "memory centre", the hippocampus, is affected by both an excess or a deficit of sleep. As yet, scientists are unsure why sleeping for too long causes a similar effect to not getting enough shut-eye.

For most of us, however, it's a definite case of "not enough". The NHS estimates up to one in three people in the UK will suffer from insomnia, with middle-aged and elderly people suffering the most.

Professor Barbara Sahakian from the University of Cambridge, one of the study's authors, said: "Getting a good night's sleep is important at all stages of life, but particularly as we age. Finding ways to improve sleep for older people could be crucial to helping them maintain good mental health and wellbeing and avoiding cognitive decline, particularly for patients with psychiatric disorders and dementia."

"Getting a good night's sleep is important at all stages of life, but particularly as we age," says Professor Barbara Sahakian from the University of Cambridge. Photo / Katarzyna Grabowska, Unsplash

During the pandemic, sleeplessness has more than doubled, from 20 per cent of the population to more than 40 per cent. More than two thirds who are in "poor health" suffer from insomnia, half of us wake to feel exhausted, and many medicate to get a decent night's sleep.

And while seven hours may be the gold standard, a further study of 2000 adults by Benenden Health found that, on average, most of us are getting just six hours a night, if we're lucky.

According to sleep expert Dr Guy Meadows – co-founder of the Sleep School - this isn't necessarily disastrous. "It is important to note that there will always be individual variation [in sleep needs], with some people needing a little less and some people needing more – a range of between six and nine hours," he says. He says that "as long as people are waking up feeling refreshed and able to cope with their day, they are probably getting the right amount".

So despite the findings of the survey, it seems "one size fits all" may not be the solution.

Dr Greg Potter, a specialist in sleep and circadian rhythms, says: "Compared to adults with 'normal' sleep duration, those who report short sleep appear more likely to develop obesity, diabetes, and cardiovascular disease." Similarly, long sleepers "tend to have a greater risk of many of the same diseases – both short and long sleep are somewhat associated with disorders of cognition".

Short sleepers

Recent studies have shown a link between sleeping for too long and dementia in older people.

But, adds Dr Potter, when it comes to the benefits of seven hours, "genetics influence sleep need. There is a very small number of people who are genetically short sleepers and need less than six hours' sleep per night."

He also points out that our ongoing health influences sleep, too. "People with inflammatory disorders or mild infections might need more sleep to help regulate activity in the immune system."

Location and season are also factors. "Your body clock is affected by when the sun comes up, so it's no surprise that people who live further from the Equator have a shorter biological nighttime," he adds.

The current National Sleep Foundation guidelines recommend that most adults aged between 18 and 64 get seven to nine hours of shut-eye per night "but the guidelines also recognise that sometimes a bit less or a bit more than these amounts is best", says Dr Potter.

For those who do get the magic seven hours though, it seems to work like a charm. Farren Morgan, 35, a physical trainer from London and member of the Coldstream Guards says, "I discovered my seven-hour sleeping regime during my training for the British military. Seven hours allows my body to fully recover from activity the day before and helps my physical performance the next day."

He also discovered that whether he slept for more or less time, his energy levels were negatively affected. His nightly regime is designed to ensure consistent sleep. "I always make sure to turn my phone on silent two hours before bed, and have no phones or TV in the bedroom." Not all of us can be so purposeful when it comes to hitting the magic seven – those with small children, ill-health, snoring partners or demanding jobs may find they have less choice. But to maximise what we do have, it's important to understand the stages of sleep, says Dr Verena Senn.

The four stages of sleep

"There are four stages of sleep that repeat throughout our sleep cycle each night and are vital for our physical and cognitive health," she explains. "Three of those stages fall into a non-REM [non-rapid eye movement] phase, and one in an REM [rapid eye movement] phase."

Stage one is non-REM sleep – "you're in a light sleep state and can be awoken very easily", says Dr Senn. "Heartbeat, eye movements, and breathing slow and your muscles will begin to relax. This stage lasts roughly 10 minutes."

Stage two, non-REM sleep, means "you're still in a light-sleep state. Your muscles become more relaxed. Bursts of brain wave activity might occur – this stage lasts between 30 and 60 minutes.

Stage three means you're in a deep sleep. "Your heart rate and breathing will be at their slowest. This stage is extremely important for your immune health and energy and your muscles will begin to restore. It's usually difficult to be woken during this stage, which lasts between 20 and 40 minutes."

Finally, stage four is REM sleep – "this usually occurs 90 minutes into your sleep cycle and is vital for memory consolidation," says Dr Senn. "That's when dreaming occurs."

For restorative rest, all stages are important. But, adds, Senn, "In your forties, new concerns about sleep can arise including sleep apnoea, decreased quality of sleep, feeling tired throughout the day, changes in hormones, and less production of melatonin.

"Elderly people tend to have a short sleep duration during the night, but strong inclinations for daytime naps. They primarily experience stage-one or stage-two light sleep and often lack stage-three deep sleep."

Studies suggest that a drop in the hormones oestrogen and melatonin may cause insomnia in some menopausal women.

According to the Sleep Foundation: "As people age, the body secretes less melatonin, which is normally produced in response to darkness that helps promote sleep by coordinating circadian rhythms."

Older people should also aim for seven hours and can maximise the chances with daytime exercise, a regular sleep schedule and avoiding alcohol, caffeine and snacking at night.

Maximising your chances for your seven hours, then, is vital. For me, reading and a hot water bottle are crucial for getting to sleep. But while individual sleep routines differ, most experts agree that avoiding caffeine after 6pm, cutting down on alcohol, not eating too late and keeping a consistent bedtime are important.

They also recommend that you don't exercise close to bedtime, keep the room well-aired and wake up at the same time each day.

When it comes to a snoring partner, however, the jury's out. Perhaps some of us can take comfort in knowing that at least our loved one is getting their seven hours' sleep – even if we're not.

Three steps to the perfect nap

1. Plan your nap, so you take it at lunchtime or early afternoon, in a quiet, private environment.

2. Set your alarm so you don't sleep any longer than 30 minutes – 20 minutes is advised.

3. After your nap, wake up promptly and do something invigorating such as washing your face in cold water so you are fully awake. - Dr Deborah Lee, sleep expert for Dr Fox Online Pharmacy