Why seven hours and 48 minutes is the sleep sweet spot

As a sleep specialist, I'm always asked how much shut-eye we need. While eight hours is what most people guess, a new study in the scientific journal Nature has uncovered that between six hours and 24 minutes and seven hours and 48 minutes is the sleep sweet spot.

Falling short, or exceeding this precise window, was linked with poorer brain, heart and immune health, according to the Columbia University scientist behind the research. It's a finding that makes sense. There is no organ in our body that doesn't benefit from sleep.

Here's why hitting the right amount is so vital for our health, and my top tips for setting yourself up to achieve it.

It helps you keep a healthy weight

Skimping on sleep will lead to weight gain . This is not surprising: it's a pattern that's been consistently found in research for years. In fact, regularly sleeping less than six hours per night was associated with obesity in the Nature paper. Another recent study, which recruited people who sleep for seven to eight hours per night – and told them to delay their normal bedtime – found that just 90 minutes less sleep than usual for six weeks led to 1lb weight gain, on average.

The main reason for this is that leptin and ghrelin – the hormones of satiety and hunger – become dysregulated when we don't get enough sleep. When they are out of sync, your body struggles to recognise when it's hungry, so it kicks into survival mode and drives up our urge to eat to ensure we don't starve.

To give you an idea of just how strong this effect is, a study found that when people sleep for less than five hours and 30 minutes, they eat an extra 385 calories the next day , equivalent to around four and a half slices of bread.

It stops you getting ill

Sleeping for too little or too long is linked with worse immune health.

When we sleep, we produce cytokines, which are chemicals that support our immune memory and defence systems (so we can recognise and fight off harmful pathogens) and also support tissue repair. If we don't sleep for the correct amount of time, these processes don't work as well and we can develop low-grade inflammation , which is a risk factor for all sorts of illnesses, from Type 2 diabetes to heart disease . 

One study neatly reflected how responsive our immune system is to sleep. Scientists recruited healthy volunteers who were exposed to a cold virus. Those who had been sleeping for six hours or less for the prior week were four times more likely to develop a cold than those who slept for more than seven hours per night.

It keeps the brain sharp

There's a phrase I like to tell my patients: sleep is of the brain, by the brain and for the brain. It's absolutely vital for our brains to function correctly, support our mental health and also lower our risk of developing dementia .

During deep sleep, waste products are cleared from our brain. If you don't get the right amount, your brain simply can't remove enough of these waste products, so they build up and we believe this contributes to dementia. 

Meanwhile, REM sleep , where most dreaming occurs, is vital for mood and memory. It's where we consolidate new memories, replenish neurotransmitters needed for sustained attention and cognitive sharpness and regulate our emotional responses.

We can even see in MRI scans that when people have consistently been lacking in sleep, there are anatomical changes in their brain. Another Nature study found that those who got less than six hours of sleep, or more than eight hours, had less grey matter, which is essential for cognitive functions like processing information.

It improves your gut

You may be surprised that your sleep and gut are linked through the microbiome – the trillions of bacteria, fungi and microbes that live in your digestive system. 

One study found that people who had too little sleep had fewer species of microbes in their gut, which is linked with worse overall health. We also know that people who have sleep disorders have altered gut microbes, a new area of research, but it seems that the microbiome impacts sleep duration and vice versa .

It supports your heart health

People who didn't get enough sleep were more likely to have high blood pressure , arrhythmias and heart disease, according to the Nature study, which shows how important sleep is for our heart health. 

When we sleep, our heart rate and blood pressure drop and our whole cardiovascular system gets a rest and time to recover. In contrast, when we're chronically sleep deprived , we miss out on those restorative processes. Research has shown that we're more likely to develop hypertension, which is the leading cause of heart attack and stroke . C-reactive protein, which is a marker of inflammation, is also elevated when we don't sleep for an optimal duration, putting us at higher risk of poor heart health.

How to improve your chances of getting the right amount of sleep

Follow a Mediterranean diet

People who eat a Mediterranean diet – rich in vegetables, fruit, whole grains and olive oil – are more likely to have better sleep quality and duration, research shows. This may be down to the amount of fibre and range of whole foods in this diet, which will nourish the gut microbiome, which we increasingly understand improves sleep.

Avoid napping

If you're having sufficient sleep at night time, you shouldn't need naps. Of course, there will be times when you can't get enough sleep over a period of nights. In that case, naps can be useful for improving your focus, making it easier to get through the day without losing concentration. 

When you do need to nap, the evidence is that it should be no more than 30 minutes. If you sleep for longer, you will get into a deep sleep and wake up feeling groggy and worse than you did before the nap.

I would advise napping in the afternoon lull, at around 3pm, as that is when we tend to feel naturally more sleepy. Avoid napping after 5pm, though, as it may make it harder to nod off that night.

Keep an eye on caffeine and alcohol

Drinking coffee, even in the early afternoon, may make you struggle to get to bed on time, as it can hang around in your system for up to 10 hours. I'd recommend having your last cup no later than midday. 

You may think alcohol helps your sleep because it makes you drowsy, but it does this at the expense of disrupting your REM sleep, which is vital for brain health.

Don't lean on sleep-promoting foods

While some studies have investigated whether specific foods promote sleep, these have been conducted over a short timeframe and in small groups who self-reported whether their sleep improved, so the evidence isn't very strong. However, there does seem to be some evidence linking tart cherry juice and kiwis with better sleep .

A much bigger impact on your sleep will be looking at your diet as a whole. Any positive effect is probably down to introducing more plant diversity. Kiwis are extremely high in fibre and cherries are very high in antioxidants .

Stick to a regular bed and wake times

One of the first things I ask patients in my sleep clinic is what time they go to bed and what time they wake up. Most can't answer. Yet having a regular sleep and wake schedule will help you sleep for the right amount of time each night. 

I recommend setting a regular wake time. Pick the time you need to be out of bed in the morning, then work backwards from there to get the ideal window of between six hours and 24 minutes and seven hours and 48 minutes of sleep. 

Aim to stick to those timings at least five nights a week, with no more than two consecutive nights veering from that schedule. 

That gives you a little bit of flexibility at the weekend. However, don't throw your sleeping schedule completely out of the window if you can help it: prolonged lie-ins every weekend are linked with poorer health.

Try not to fall asleep on the sofa

You have a finite amount of sleep that you need in a 24-hour period and if you've had an hour of it on the sofa, that's an hour that you're not going to have in your bed. It can leave you struggling to fall asleep once you get into bed and contribute to waking up in the night.

Don't be a slave to sleep props

I would avoid having too many sleep props (accessories to get to sleep). It shouldn't be the case that we need earplugs , an eye mask , a completely blacked-out room and white noise machines to sleep well. The exception is if you have an environment that stops you sleeping, like noisy neighbours or bright street lights outside your window.

Using sleep props can become counterproductive, because you can persuade yourself you can't sleep without them and, of course, that's not true. If you're not using a sleep prop now, don't start using one.

Set strict phone rules

You've probably heard that the blue light from your phone and other screens disturbs sleep, but the much bigger issue is simply that it eats into the time you would otherwise be sleeping. You think you're going to quickly look at your phone and suddenly, an hour later, you're still on it checking emails or scrolling through social media.

I'd recommend that you stop using your phone 60 minutes before you intend to sleep, and keep it outside your bedroom so that you're not tempted to scroll late into the night . 

My own sleep routine is very simple. I have an early dinner with my children and avoid eating a heavy meal within two hours of bedtime. I wind down by watching television or reading in the evening. I also switch to lamps rather than switching on overhead lighting to signal to my brain that it's nearly bedtime. 

I stop using my phone an hour before bed. Just before it's time to sleep, I wash my face and brush my teeth to provide further signals that it's time for bed, and I keep my bedroom relatively dark, but not pitch black, at night.

If you're doing all of this and you're still not able to feel adequately rested, see your GP or a sleep specialist. Otherwise, happy sleeping.

As told to Emily Craig

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