There’s nothing like a good night’s sleep, but not all snooze time is created equally. The deepest stage – also known as slow wave sleep – is thought to be the most restorative, playing a crucial role in memory, growth and immunity.
“If you look at brain waves during the deeper stages of sleep, they’re characterised by big, slow waves, which is why deep sleep is known as slow wave sleep,” says Dr Neil Stanley, author of How To Sleep Well: The Science of Sleeping Smarter, Living Better and Being Productive. “The reason it’s so crucial is because it’s where most of the important stuff takes place. Put simply, it’s when your brain does its housekeeping.”
Dr Stanley says deep sleep is the time your brain lays down memories and learns new tasks, and it’s when our immune system becomes optimised. “This stage of sleep is also when young children physically grow, which is why they need so much. And the reason why we feel so groggy, awful, and disorientated when we’re suddenly woken from a deep sleep is a protective measure, because it’s so important for our health.”
A 2023 study published in the journal JAMA Neurology found that not getting enough of this deep sleep can impact your long-term brain health, and even increase the risk of dementia. “We found that ageing was associated with a decline in the amount of the deepest stages of sleep” said study author Matthew P Pase, an associate professor of psychologist and neurology at Monash University in Australia, whose research found that those who got the least slow wave sleep had a higher risk of dementia during its 17-year following up period.
“Poor sleep in general is associated with increased anxiety, and some recent studies have suggested that slow wave sleep might be important in organising brain networks in the prefrontal cortex to reduce anxiety,” says Russell Foster, professor of circadian neuroscience at the University of Oxford, and author of Life Time: The New Science of the Body Clock. A lack of sleep has also been linked to reduced immunity, heart disease, diabetes and mood disorders.
Yet many of us aren’t getting enough of it; a 2023 study from Nuffield Health found that on average, British people are only getting 5.19 hours a sleep per night, compared to the 7-9 hours per night the NHS recommends. The same research also found that of those surveyed, only 36 per cent said their sleep was “good”. However, when it comes to studies on how much sleep we’re getting, it’s often hard to untangle how much of that is the deep stage.
“What’s interesting about SWS,” says Professor Foster, “is that it’s protected and defended by the body because it’s so important for our brain health. For example, if you’re sleep-deprived, the brain will still ensure that when you do sleep you’ll get enough. So even if you don’t get enough sleep overall, your brain will make sure you get SWS when you next fall asleep.
“Most slow wave sleep occurs during the first half of the night, and this may be the basis of the often quoted saying that ‘an hour of sleep before midnight is worth two hours of sleep after midnight’. But for what it is worth I think this is just another sleep myth,” says Professor Foster.
He adds that slow wave sleep is linked to the ability to remember facts, where most of our brain’s memory consolidation takes place, along with the processing of information.
The other stages of sleep are the light stage when you first fall asleep and can be easily roused, and the REM (rapid eye movement) phase, which is also deep and when you dream, and is associated with emotional regulation.
So, if the holy grail of sleep is deep sleep, how can we get more of it?
Certainly not through alcohol, says Professor Foster, who says that despite its reputation as a sedative, alcohol actually reduces the amount of deep sleep we have. “Those who drink heavily tend to get less slow wave sleep than most, which is why memory and processing is so bad in alcoholics.
“The truth is, we get the amount of deep sleep we need naturally, because our body defends against losing it. Sadly there is no magic bullet for improving your deep sleep, other than improving your overall sleeping habits and getting more sleep in general.”
He points to the work of Dr Phyllis Zee, a professor of neurology at Northwestern University Feinberg School of Medicine in the US. “There is a lot of work going on there currently with experts like Dr Zee using sound waves and frequencies to stimulate the brain to go into deep sleep, which she believes can lead to greater memory consolidation in older people. It’s still early days, but her work is very interesting.”
From middle age onwards, deep sleep begins to decrease, which scientists believe may contribute to brain ageing and memory loss. In one small study of 13 adults, Dr Zee found that playing a waterfall-like sound enhanced deep sleep in the study participants, and she called it a “potential tool for enhancing memory in older populations and attenuating normal age-related memory decline.” Although she said more research is needed.
“Really though, there’s no way of getting more deep sleep other than getting more sleep overall,” says Neil Stanley. “And this is particularly important for older adults.”
Dr Stanley says we begin to lose deep sleep as we get older: “This happens from the ages of 35 to 40 in men and 50 in women. As we get to our fifties and sixties our sleep feels less refreshing because, while we may sleep the same amount of time, less of that sleep is made up of deep sleep.”
One theory for this is that the older we get, the fewer novel experiences we encounter: “If you think about it, slow wave sleep is associated with memory and retaining information,” says Dr Stanley. “Babies and children are constantly learning, so one could argue that if you are experiencing lots of new things you’ll get more slow wave sleep. Will doing puzzles, learning a new language, visiting new places, or meeting new people lead to more deep sleep? My guess is it will.”
Lastly, nap wisely. “Data shows the perfect nap time is around 20 minutes,” says Professor Foster, as this will increase alertness and concentration in the afternoon. “Any longer and the nap will become counterproductive and affect your ability to fall asleep, as will naps too close to bedtime.”