We usually judge a good night’s sleep by the number of hours we get. But the time we spend in different stages of sleep could be just as important.
In a study of almost 100,000 British adults, researchers found that people who spent more time in REM sleep (the stage most closely associated with vivid dreams) were less likely to develop 83 different health problems in later years. These ranged from heart failure to dementia.
But there is a catch.
A week of wrist movements, followed by years of health records
Measuring sleep on a large scale is very difficult. The best-known method, called polysomnography, records brain waves, eye movements and muscle activity to distinguish stages of sleep.
However, the laboratory’s equipment and setup at nighttime make it difficult to use with tens of thousands of people over extended periods of time.
Sleep questionnaires offer a simpler alternative, but people may misremember their sleep duration or fail to notice brief awakenings, so the method isn’t really reliable. People also can’t tell how much time they spent in different stages of sleep.
To get a more objective picture, researchers from Peking University and Capital Medical University in Beijing used data from the UK Biobank, a long-term health study. Between 2013 and 2015, participants wore wrist accelerometers for seven consecutive days. These devices recorded movements, which researchers could use to estimate sleep patterns.
Of the 103,611 participants who returned usable records, the team kept data from 95,559 people (the rest failed quality checks or did not meet the study’s completeness requirements). The average age of participants was 56.2 years.
The researchers then used a machine learning system called SleepNet to estimate participants’ sleep stages from their wrist movements. The algorithm identified patterns associated with REM sleep, non-REM sleep, and wakefulness.
REM sleep occurs when the brain becomes particularly active and the most vivid dreams occur. Non-REM sleep includes lighter stages and deep sleep, associated with physical restoration and recovery. Over the course of a typical night, we ride between these stages several times.
The team compared these measurements with hospital patient records covering 1,049 health outcomes. Participants were followed for a median period of 8.9 years, with follow-up ending at the first recorded onset of the relevant disease, death, or April 1, 2024.
The researchers also took into account factors such as age, gender, body mass index, smoking, alcohol consumption, physical activity and environmental exposures.
REM sleep has stood out in dozens of diseases
The analysis identified 156 significant associations between sleep patterns and newly recorded diseases.
People who spent more time in REM sleep had a lower risk of developing 83 different diseases. These included cardiovascular diseases such as heart failure, high blood pressure and ischemic heart disease, as well as neurological diseases such as dementia, Parkinson’s disease and multiple sclerosis.
Some associations were important. Comparing participants whose estimated REM sleep differed by approximately 48 minutes per night, those who had more REM sleep had a 26% lower relative risk of heart failure and a 46% lower relative risk of dementia during follow-up. These numbers describe statistical associations and not the benefits a person might expect from deliberately increasing their REM sleep.
It’s important to note that it’s not just about how much people sleep in general; The REM seems to be important despite everything. When researchers took total sleep time into account and looked at REM sleep as a proportion of sleep time, the associations with 54 conditions remained statistically significant.
Deep sleep also appears to play an important role, although for fewer illnesses. Higher amounts were associated with a lower risk of seven conditions, including type 2 diabetes, major depressive disorder, sleep apnea and Parkinson’s disease.
That said, sleep duration also matters. The researchers found relationships between sleep duration and 86 disease outcomes, meaning the relationship did not follow a simple increasing or decreasing trend. For 69 of these conditions, the estimated least risk sleep duration fell within a window of six to eight hours.
People sleeping less than five hours had the broadest pattern of adverse associations: They accounted for 37 of the 41 significant adverse associations identified when extremely short sleepers were compared to the six- to eight-hour reference group. Longer sleep duration was also associated with a lower risk of 50 conditions.
Better sleep measures, but no evidence of cause and effect
The size of the study and the long follow-up offer valuable clues about the relationship between sleep and health.
However, it cannot establish that changing a person’s sleep stages will prevent the disease. Existing health conditions could themselves disrupt sleep, while other unmeasured factors (such as stress and diet) may influence both sleep and subsequent diagnoses.
There is also a technical limitation. Wrist movements are not the same as directly measuring brain activity. Although SleepNet was trained and validated against laboratory sleep recordings, its ability to distinguish REM sleep, non-REM sleep, and wakefulness from movement data was only moderate.
The algorithm’s REM estimates differed from laboratory measurements by about 17 minutes on average, with other systematic differences also observed. And because participants wore the devices for only a week, the recordings may not fully represent their typical sleep patterns over several years. This is an important limitation of the study.
The study also relied on hospitalized patient diagnoses rather than all medical records, and its participants were overwhelmingly white, which could limit how broadly the results can be applied.
Yet the research adds to growing evidence that sleep has more dimensions than just the number of hours spent in bed.
For now, though, there’s no reason to start obsessing over the REM percentages displayed by a smartwatch. The most helpful message remains familiar: get enough sleep, stick to a reasonably consistent schedule, and see a doctor if sleep problems persist.
The study is published in the journal PLOS.
Gn Health
