We’re often told we need to get eight hours sleep a night, but less focus is put on the quality of the sleep you’re getting – and how deeply you’re sleeping. 

Sleep is made up of three parts: light sleep, deep sleep and dreaming sleep. All of these are needed to be healthy. “As we sleep, we go from light to deep to dreaming sleep in approximately 90-110 minute cycles which repeat across the night,” sleep expert Dr Lindsay Browning tells HuffPost UK.

When you’re in a deep sleep, it’ll be pretty hard for you to wake up – and if you do stir within that timeframe, you’ll feel super groggy.

While deep sleep is crucial for our bodies to function day-to-day, it’s only one piece of the pie, says Dr Browning, author of Navigating Sleeplessness: How To Sleep Deeper And Better For Longer. If you only have deep sleep and no light sleep or dreaming sleep, you won’t get all the benefits of shut-eye. 

But it’s still needed. Deep sleep helps you feel refreshed the next day and has functions like repairing the body and memory consolidation. And young children need much more of it than older kids and adults. “It’s the part of sleep where we secrete growth hormones – vital for growing children,” says Dr Browning. 

So, how much deep sleep should we be getting? A healthy young adult will spend around 18% of the night in deep sleep, 23% in dreaming sleep, 54% in light sleep and 5% awake. If you sleep for eight hours, that’s between one and two hours in deep sleep each night.

If you’re not getting as much as you’d like – a sign is waking up exhausted all the time – Dr Browning says you should give yourself more time to sleep. This means getting into bed earlier and not setting an alarm for as early as normal – simply let your body get the rest it needs.

Some studies suggest if you exercise more during the day, you’ll fall asleep quicker and have more deeper sleep that night, she adds. Try aerobic activities like jogging, running and swimming – but don’t do these too close to bedtime, as they might have the opposite effect and wake you up. 

Caffeine reduces the amount of deep sleep you will have, so if you’re a bit of a coffee addict, avoid drinking tea, coffee or cola at least six hours before bed. And one study from the US suggests a warm bath or shower taken one or two hours before bedtime can also improve your sleep quality. 

As we get older, the percentage of time spent in deep sleep decreases and the percentage spent in light sleep increases. This reduction tends to be greater in men than women. This is all a natural part of ageing, so don’t spend time worrying about it, as “it’ll only make you less likely to sleep in the first place”.

If you’ve tried the tips above and are still waking up feeling groggy and unrefreshed, Dr Browning says a sleep disorder might be at the root of the problem. Sleep apnea – where breathing repeatedly stops and starts throughout the night – is one of the biggest issues preventing deep sleep, she says. 

“When people have sleep apnea, they experience repeated awakenings throughout the night,” she explains. “These repeated awakenings disrupt your sleep cycles and result in reduced time spent in deep sleep.”

The issue often remains undiagnosed as a person with sleep apnea may not recall waking up during the night. They might report sleeping for a long time, but wake feeling unrefreshed due to the disturbed quality of their sleep. If this sounds familiar, it’s best to talk to your GP.

Dr Lindsay Browning is the author of Navigating Sleeplessness: How to Sleep Deeper and Better For Longer (published by Welbeck Balance, £8.99) and a chartered psychologist at Trouble Sleeping.