What is core sleep and how much do you need?

Core sleep is needed for everything from improving memory to relieving stress. Here’s how to get more of it

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Are you bored of being told that you need to work on your core? Well, you do. But here’s the good news. The best exercise regime involves lying down in a dark room, pulling a blanket over your torso, closing your eyes and…having a snooze; because your stomach, pelvis and hips are not the only core that counts.
The concept of “core sleep” is now also gaining credence. The quality and quantity of yours may influence everything from your immunity and hormonal regulation to your ability to solve problems, process information and regulate your emotions.
To understand core sleep, first you need to get your head around the sleep-wake cycle, says Prof Russell Foster, the director of the Sleep and Circadian Neuroscience Institute at the University of Oxford, and author of Life Time: The New Science of the Body Clock.
Every night, and unwittingly, you complete a series of non-REM and REM sleep cycles, he explains.
From quiet rest, you descend into three consecutive stages of non-REM sleep: one, two and finally three, as pictured below. As Prof Foster explains: “Brain waves (or electrical activity) go from a state of ‘very small amplitude, very high frequency’ to one of ‘much bigger amplitude, lower frequency’.” Once you reach that final stage – three – you are considered to be in deep sleep.
But that is not the end of your journey through the land of nod. From stage three, you move into REM (or “rapid eye movement”) sleep, before rising up through the non-REM stages again. Then, repeat.
Each of those cycles takes about 90 minutes, explains Prof Foster, though the timings evolve as we age. Normally, you might expect to go through five cycles in a single night.
Just to confuse things, Apple has a completely different definition, all of their own. The sleep tracker on their watches breaks your slumber into three supposed stages: REM, core and deep. According to Prof Foster, however, the most sensible way of thinking about core sleep is “the minimum amount of slow wave [or non-REM, stage three] and REM sleep you need for optimum function”.
Dr Guy Meadows, the founder and clinical director of The Sleep School and creator of the Sleep School App, agrees. He says: “Core sleep includes the most restorative stages of sleep, primarily deep sleep (non-REM, stage three) and REM sleep, which are crucial for physical and cognitive health.”
Sleep, as highlighted below, takes four different stages.
There are numerous health benefits to core sleep. These include:
Non-REM stage three and REM sleep have particular and important roles to play in our health and wellbeing.
“It looks as though, during deep sleep or non-REM stage three, we get consolidation of memory,” says Prof Foster. “The facts we’ve been exposed to during the day are laid down into memory.”
During this phase in our sleep we are also processing information, he explains. Though you are blissfully unaware of this work, your brain is capable of coming up with innovative solutions to complex problems in this state.
Research bears this out, says Prof Foster: “A night of sleep has been shown to hugely enhance our ability to solve problems. If you introduce a problem on one day, and ask people to solve it the next, those that slept have a much better chance than those that didn’t.”
In 2023, researchers from the University of California, Berkeley found that this sleep stage can also act as a “cognitive reserve factor”, potentially increasing your resilience against a protein in the brain (beta-amyloid) linked to memory loss in dementia. “Interestingly, those with schizophrenia don’t show any stage three sleep at all,” says Prof Foster. “And that could be related to their impairment in cognition.”
Different, but equally important, things are happening when you move into the next sleep stage: REM.
It is in this phase that we have our most vivid and complex dreams, explains Prof Foster: “and it looks as though that’s when we’re processing our emotional experiences.” Again, studies bear this out. “People deprived of REM sleep will show higher levels of anxiety,” he says. Research from Imperial University suggests that people who suffer from PTSD experience less REM sleep, contributing to the theory that REM sleep helps us process difficult emotions and stress.
Good core sleep is not, however, only important for your brain. It is crucial for your physical health too, suggests Dr Meadows: “it enhances immune function, promotes muscle repair and growth, and regulates hormones. Core sleep, particularly deep sleep, is crucial for physical restoration and recovery.”
“Several factors can influence the quality of core sleep, encompassing environmental, physical and emotional/mental health aspects,” says Dr Meadows. “Luckily, some of those environmental contributors may be within your control and can significantly enhance sleep quality.” These include:
On the flip side, physical conditions like sleep apnea or chronic pain can also disrupt sleep patterns. Dr Meadows explains that while “emotional and mental health factors, such as stress, anxiety, and mental health disorders, also play a significant role – high stress or anxiety levels can make it difficult to fall asleep, while disorders like anxiety and depression can lead to fragmented sleep and reduced sleep quality”.
Ironically, some of the strategies commonly used to self-medicate in such situations can add to the problem, suggests Prof Foster. “Some medication, like sleeping tablets, will affect your level of deep sleep negatively, and alcohol suppresses REM sleep,” he explains. “That’s why alcoholics and borderline alcoholics have poor memory consolidation and poor processing of information.”
“Improving the quality and quantity of core sleep involves employing several strategies,” says Dr Meadows.
First, he suggests we should be “maintaining a regular sleep-wake cycle by going to bed and waking up at the same time every day – even on weekends – [it] helps regulate the body’s internal clock.”
Prof Foster, a world-expert on circadian rhythms, agrees. It is also a good idea to get outside and into natural light first thing in the morning, and again at dusk, he suggests.
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Then, of course, there is diet. “Eating a healthy diet, leaving at least three hours between eating and sleeping, and limiting stimulants such as caffeine, alcohol and nicotine can also improve sleep quality,” stresses Dr Meadows.
Finally, the elephant in the bedroom: stress. “Managing daytime work stress and anxiety through conversations with colleagues, taking regular breaks and practising self-kindness can further enhance your ability to sleep well,” says Dr Meadows.
The vast majority of people (around 80 per cent) need seven to eight hours of sleep, with almost all of us (97 per cent) needing somewhere between six and nine hours, says Dr Meadows.
Some researchers have tried to put a figure on how much of this should be “core sleep” stating that around 25 per cent of your sleep should be deep sleep, and another 25 per cent REM sleep.
The amount of overall sleep we need is dependent on our age, as shown in this graph below:
Even a 1 per cent annual reduction in the deep sleep you get can have profound effects, according to research published last year which found that it translated to a 27 per cent increased risk of dementia.
But do not panic, says Prof Foster: “Overall, sleep is the most important cognitive enhancer that we know. But we can’t really measure core sleep without going into the lab, so I think, for the vast majority of people, it’s more useful to think about the overall sleep we need.”
It is simple, says Prof Foster: “Ask yourself the obvious questions: do you wake easily? Do you need coffee to stay awake? Are you irritable or making bad decisions? Those are the valuable things to measure.”
Yet core sleep is not the only component that counts. Non-REM stage one sleep has recently been shown to help in more than relaxation, says Prof Foster: “it turns out that you consolidate memory and process information too. A nap, for example, when you’re not going into deep sleep, and stay in stages one and two instead, can really help your ability to function in the later stage of the day. Which tells us that something important is going on during those stages too.”
In fact, if you decide to take a nap: “You’re actively advised not to go into a deep sleep because recovery from that deep state can leave you groggy,” he says.
Besides, the science of sleep is very personal. The amounts needed differ significantly from person to person, and as we age, we may possibly need less core sleep.
Babies and teenagers get far more of it, says Prof Foster. “That starts to change noticeably in the 20s and into the 30s,” he explains. “The question is: why? If you agree with the idea that the slow wave sleep is all about consolidating memory and processing information, then it makes sense really. When you’re young, everything you see, every day, is new. All these new experiences need to be stored and processed. But, as you age, you’re encountering less new information. You’ve seen it all before. So perhaps we don’t need as much deep, slow wave sleep.”
Don’t let that put you off having a snooze, though.
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