This kind of sleep is essential for a healthy brain. Missing out can be devastating

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 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, 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 a better night’s sleep

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, Malkani said.

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 said.

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, said Bryce Mander, an associate professor of psychiatry and human behaviour at the University of California, in Irvine.

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Having a consistent sleep and wake time can help you fall asleep more easily, said Zsófia Zavecz, a postdoctoral researcher at the Adaptive Brain Lab at the University of Cambridge, in the United Kingdom.

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”, like learning a new skill, could exhaust certain parts and increase their need for restorative, slow-wave sleep, Zavecz said.

Exercising can keep you mentally engaged and increase blood flow to the brain, which is helpful in glymphatic clearance, Nedergaard said. Minimising stress also boosts the process, she added.

So how do you know if you’re getting enough sleep? Wearable trackers or smartphone apps can estimate the amount of time you spend in each cycle, but Malkani said 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, Pase said. “Let the brain do its thing, and it will shuffle around as it needs,” he said.

This article originally appeared in The New York Times.