A good night’s sleep isn’t just about the number of hours you log. Getting quality sleep – the kind that leaves you feeling refreshed and ready for the day – is critical for a healthy brain.
People with disturbed sleep, because of something such as insomnia or sleep apnoea, have a higher risk of developing dementia than those with no sleep issues. Poor sleep can harm your brain in other ways, too. One study found that people in their 30s and 40s with heavily disrupted sleep (such as frequent awakenings or movements) were two to three times more likely to test lower in executive function, working memory and processing speeds a decade or so later.
Scientists think that deep sleep and rapid eye movement (or REM) sleep are particularly influential when it comes to brain health and dementia risk. A study published this year on people with deep sleep and REM deficiencies found that the subjects’ brains showed signs of atrophy in MRI scans 13-17 years after the deficiencies were observed; the atrophy looked similar to what you’d find in the early stages of Alzheimer’s disease.
What scientists know
When you’re asleep, your brain continuously cycles through four distinct phases: two stages of lighter sleep, when your body relaxes and your heart rate and temperature drop; deep sleep or slow-wave sleep, when brain activity slows; and REM, when you typically dream. The brain generally takes about 90 minutes to cycle through all four stages and then restarts the process.
Deep sleep and REM help your brain “heal itself” from fatigue and stress and consolidate memories, said Matthew Pase, an associate professor at the School of Psychological Sciences at Monash University in Melbourne, Australia. In deep sleep, your brain regulates metabolism and hormones; it also acts as a “rinse” for the brain, clearing out waste. REM is when your brain processes emotions and new information you picked up when you were awake.
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The two phases influence dementia risk in different ways, scientists think.
As part of the rinsing process in deep sleep, your brain flushes out amyloid proteins that are a hallmark of Alzheimer’s. Years of interrupted deep sleep and incomplete flushing – known as glymphatic failure – could hasten the onset of dementia, said Dr Maiken Nedergaard, a neurology professor who researches the glymphatic system.
Scientists understand less about how REM is tied to dementia risk, said Dr Roneil Malkani, an associate professor of sleep medicine.
A 2017 study of more than 300 people over 60 found that a shorter amount of nightly REM sleep, and taking longer to get to the REM phase in each sleep cycle, were both predictors for dementia later in life. That could be because REM is “vitally important” for storing and processing memories, and losing that capacity both weakens the brain’s defences against cognitive decline and can accelerate atrophy in parts of the brain that aren’t used, said Dr Pase, who co-wrote the study.
It’s also hard to tease out the “chicken and egg” relationship between sleep and dementia, and whether poor sleep definitively causes it, Dr Pase said. Adults (particularly women) naturally spend less time in deep and REM sleep as they age. Scientists already know ageing itself increases dementia risk, but dementia also tends to worsen sleep. It’s possible the two processes “compound each other,” he said.
Tips for better sleep
Sleep is as important for survival as food and water, says Dr Maria Pertl, health psychology lecturer at the Royal College of Surgeons in Ireland (RCSI). “If we haven’t had a good night’s sleep, we can’t pay attention to things properly and we have trouble focusing,” she says. “We tend to be more irritable as our mood fluctuates.”
While Dr Pertl outlines the important physical and mental health benefits of sleep – everything from cell repair to protecting the body from infections, consolidating learning and long term memories – she emphasises how we shouldn’t worry if we miss a bit of sleep from time to time. “Worrying about not getting enough sleep is counterproductive as the body isn’t able to relax,” she says. Aiming to rest and not necessarily to get to sleep can be helpful.”
It’s generally difficult to target individual stages of sleep for improvement, and as you get older, experts think it may be harder to change the brain’s sleep cycles. But there’s no downside to improving your sleep hygiene, which is an effective way to boost your sleep overall, including deep and REM sleep, says Dr Malkani.
Getting about seven hours of sleep a night is the easiest step you can take. That gives your brain enough time to cycle through its stages between four and seven times, he says.
Research has shown that people who sleep six hours or less a night in their 50s, 60s and 70s have a 30 per cent increased risk of dementia later in life, suggesting that it’s never too late to improve your sleep, says Bryce Mander, an associate professor of psychiatry and human behaviour.
Having a consistent sleep and wake time can help you fall asleep more easily, says Zsófia Zavecz, a postdoctoral researcher at the Adaptive Brain Lab at the University of Cambridge. What’s more, parts of the brain that are used heavily during the day tend to exhibit slower brain waves during sleep, so doing anything that “meaningfully engages the brain for a while,” such as learning a new skill, could exhaust certain parts and increase their need for restorative, slow-wave sleep, he adds.
Exercising can keep you mentally engaged and increase blood flow to the brain, which is helpful in glymphatic clearance, says Dr Nedergaard. Minimising stress also boosts the process, she adds.
Getting enough sleep
Wearable trackers or smartphone apps can estimate the amount of time you spend in each cycle, but Dr Malkani says it’s more helpful to ask yourself: “How do I feel when I wake up?” And if you wake up in the middle of the night, ask: “How long did it take me to fall back asleep?”
In general, setting aside enough time to sleep is the best way to ensure your brain reaches deeper stages – and depending on deficits, it may spend more time in REM or deep sleep as it cycles, says Dr Pase. “Let the brain do its thing, and it will shuffle around as it needs,” he says.
Getting to sleep
Dr Richard Costello, consultant in respiratory and sleep medicine at Beaumont Hospital, Dublin, and professor of respiratory medicine at the Department of Medicine at the RCSI suggests “sleep restriction” as a counterintuitive tip to getting a good night’s sleep. “It’s about not getting into bed and having a fight with yourself about [struggling to get to sleep]. Go to bed when you are really tired and you won’t be fretting, tossing and turning in bed,” he says.
“Do something dull and repetitive or read a book and then go back to bed,” he says.
Dr Pertl says that listening to a classical fiction audiobook has been a really effective of helping her get to sleep. “It stops me thinking of things that keep me awake but the quiet, calm voice is not too engaging so I fall asleep quickly.”
