48 Extra Minutes of REM Linked to 46% Lower Dementia Risk

Most of us judge our sleep by one number: how many hours we got.

But a night of sleep isn’t one thing. It’s a cycle of light sleep, deep sleep and dreaming REM sleep, repeated several times.

A huge new study asked whether those different parts of sleep are linked to different diseases years later.

It followed almost 100,000 people who wore a wrist tracker for a week. The researchers then checked their hospital records for more than 1,000 diseases.

The results point to two things: how much REM sleep you get, and whether you land in a 6 to 8 hour window.

The study: 95,559 people and a wrist tracker

Who took part

The researchers, from Peking University in Beijing, used the UK Biobank. That’s a long-running health study of half a million British adults.

Between 2013 and 2015, a subset wore a wrist accelerometer for seven days. After quality checks, 95,559 people were included.

Their average age was 56 when they wore the device. Almost all identified as white.

Reading sleep stages from movement

The tracker records movement, not brain waves. So the team used an artificial intelligence model called SleepNet to estimate sleep stages.

SleepNet had been trained on more than 1,100 nights where people wore both a tracker and a full lab sleep test.

It’s a clever shortcut, but it isn’t perfect. Its agreement with lab sleep tests was only moderate.

What they tracked

The researchers followed everyone for a median of 8.9 years. They looked for any of 1,049 conditions in hospital records.

They also adjusted for many other factors, including age, lifestyle and where people lived.

A typical night in the study

Here’s what an average participant’s sleep looked like, based on the tracker.

Most people fell asleep between 9 and 11 pm.

How much REM sleep do you need?

There’s no official target for REM sleep. Health agencies advise on total sleep, not on sleep stages.

Here’s what is known. Sleep runs in cycles of about 80 to 100 minutes, usually four to six a night.

You get more REM sleep later in the night. That’s according to the US National Heart, Lung, and Blood Institute.

In this study, REM made up about a fifth of the night. The average participant got about 82 minutes.

People sleeping 6 to 8 hours averaged about 92 minutes. Short sleepers averaged about 65.

So the practical answer isn’t a number to chase. Enough regular sleep gives your brain time to reach those longer REM periods near morning.

The results: REM sleep stood out

156 links after strict checks

With over 1,000 diseases tested, some links would appear by chance. So the team used a very strict statistical correction.

After that, 156 links between sleep and later disease remained.

More REM, lower risk

REM sleep had the most links by far. People with more REM sleep had lower risks of 83 diseases across 12 categories.

These are the comparisons between people with more versus less REM, about 48 minutes apart.

Deep sleep had fewer links, but they included type 2 diabetes, depression and Parkinson’s disease.

Light sleep was more mixed. More of it was linked to higher risk of depression, but lower risk of heart failure.

The authors think that depends on context. Extra light sleep as part of a longer night may be fine, while a night dominated by light sleep may signal poor-quality rest.

Broken and irregular sleep

Two other patterns were linked to higher risk.

People whose sleep length varied a lot from night to night had higher risks of depression and anxiety. Those with more time awake during the night had higher risks of problems such as alcohol misuse.

The 6 to 8 hour window

Sleep length had a curved relationship with 86 conditions. Too little and too much were both linked to higher risk.

For 69 of them, the lowest risk fell between 6 and 8 hours a night. For type 2 diabetes, it was about 7.3 hours; for dementia, about 7.5.

Under 5 hours was the danger zone

The team also compared people sleeping under 5 hours with those sleeping 6 to 8.

The short sleepers accounted for 37 of 41 significantly higher risks. For example, their risk of dementia was about 2.6 times higher, and of repeated falls about 1.8 times higher.

Why 6 to 8 hours might matter

People sleeping 6 to 8 hours also had healthier-looking sleep.

The middle group had the highest share of REM sleep. Short sleepers had far less REM and much more irregular nights.

Why might REM and deep sleep matter?

REM sleep is when most vivid dreaming happens. It’s thought to help with memory, emotions and brain health.

Deep sleep is linked to physical recovery, blood sugar control and possibly clearing waste from the brain.

The authors suggest short sleep may strain the body through stress hormones, inflammation and an overactive nervous system. Those ideas fit the heart and brain links, but this study can’t prove them.

The catch: sleep can be an early symptom

Here’s the part I think matters most. Many diseases change sleep years before they’re diagnosed.

Parkinson’s disease, for example, can disturb sleep years before its familiar symptoms appear.

The researchers checked for this. They ignored diseases diagnosed in the first two years after the tracker week.

About 60% of the links survived (95 of 156). That means some links may reflect illness already brewing, rather than poor sleep causing disease.

What earlier research found

Sleep length and death

A 2019 meta-analysis in Epidemiology and Psychiatric Sciences pooled cohort studies on sleep duration and death.

It found long sleep, over 9 hours, was linked to higher mortality. Short sleep under 7 hours showed no clear difference in that analysis.

The authors noted long sleep may partly reflect underlying illness.

Less REM, higher mortality

A 2020 study in JAMA Neurology used lab sleep tests in 2,675 older men, followed for about 12 years.

Every 5 percentage points less REM sleep was linked to a 13% higher death rate. The finding was repeated in a younger group of men and women in Wisconsin, followed for about 21 years.

In that study, REM sleep was the sleep stage most closely tied to survival.

How it fits together

The new study agrees that REM sleep seems to matter, and extends it to dozens of specific diseases.

It also suggests the healthiest range may be slightly shorter than the usual 7 to 9 hours advice, at least for middle-aged and older adults.

How much should you trust this?

Promising. It’s big and long-running, with objective sleep data, but it can’t show that sleep causes disease.

What makes it convincing

• Nearly 100,000 people wore a tracker, rather than guessing their own sleep.
• It followed them for almost nine years.
• It used a very strict correction for testing so many diseases.
• Most links held after adjusting for sleep disorders, other illnesses and season.
• The authors declared no competing interests.

What makes me cautious

• Sleep stages were estimated from wrist movement, with only moderate accuracy.
• Sleep was measured for just one week.
• Only about 60% of links survived the two-year check for early illness.
• Diseases came from hospital records only, so milder cases were missed.
• UK Biobank volunteers are healthier and wealthier than average, and mostly white.

What this means for you

You can’t easily choose how much REM sleep you get. But you can protect the conditions that let your body cycle through sleep properly.

• Aim for enough sleep. For most adults, that means at least 6 to 8 hours, and many people need more.
• Keep a regular schedule. Similar bed and wake times help your sleep stay stable.
• Don’t panic over one bad night. These links are about long-term habits, not the odd short night.
• Take wearables with a pinch of salt. They estimate sleep stages; they don’t measure them like a lab test.
• Talk to a doctor about ongoing problems. Snoring, gasping, or waking often could signal a treatable sleep disorder.

The NHS has practical tips on how to fall asleep faster and sleep better, and advice on insomnia.

Poor sleep and low mood often go together. If you’re in crisis, please contact a person, not a chatbot.

In the US, call or text 988. In the UK and Ireland, you can call Samaritans on 116 123, any time.

Elsewhere, Find a Helpline lists free services. If someone is in immediate danger, call emergency services.

In this TED video, sleep scientist Matt Walker walks through the stages of a night’s sleep:

What we still don’t know

1. Does more REM sleep protect health, or just reflect it? Only studies that change sleep can tell.
2. Can we increase REM sleep on purpose? Safe, reliable ways haven’t been proven.
3. Would lab-quality sleep data change the picture? Wrist trackers blur sleep stages.
4. Does one week of sleep represent years? Habits shift with age, work and health.
5. Do the results hold in more diverse groups? Most participants were white and middle-aged or older.
6. Is 6 to 8 hours right for younger adults? This study didn’t include them.

My take: protect your sleep, not your sleep score

What I like about this study is its scale. It maps sleep against more than 1,000 diseases, instead of cherry-picking one.

I’m less convinced by the precise numbers for single diseases. Sleep stages from a wrist tracker are rough, and some links probably reflect illness that had already started.

Still, the big picture is consistent with years of research. Very short sleep is a warning sign, and a steady 6 to 8 hours looks like a sensible target.

I wouldn’t chase REM minutes on an app. I’d focus on the basics that make good sleep possible.