How much we sleep is important, but how our sleep is structured may also matter. A large study of more than 95,000 people found links between the duration of REM and deep sleep and a lower risk of dozens of diseases, while sleep of less than five hours was associated with the worst health profile.
The study was published on September 17, 2026 in the scientific journal PLOS Medicine. Scientists analyzed data from 95,559 participants in the UK Biobank, followed for an average of 8.9 years.
The participants wore accelerometers on their wrists for seven days. Using the SleepNet algorithm, researchers assessed REM, light and deep sleep, total duration, nighttime awakenings and sleep regularity. These metrics were then compared to the occurrence of 1,049 health conditions. The algorithm was previously validated against laboratory polysomnography, but is not equivalent to it.
More REM sleep, lower risk of dozens of diseases
The most striking association was found with REM sleep, the phase in which brain activity is high and we usually dream the most intensely. An additional 47.6 minutes of REM sleep was associated with a lower risk of 83 diseases.
Among them are heart failure, atrial fibrillation, ischemic heart disease, dementia and Parkinson's disease. For example, for dementia, the risk ratio is 0.54, and for heart failure, 0.74. These are statistical associations and do not mean that increasing REM sleep by a certain number of minutes will necessarily reduce an individual's risk by the same percentage.
Deep sleep also stands out. Its longer duration is associated with a lower risk of seven diseases, including type 2 diabetes, major depressive disorder, sleep apnea and Parkinson's disease.
The study found a nonlinear relationship between sleep duration and 86 diseases. For 69 of them, the lowest statistical risk was concentrated mainly between 6 and 8 hours of sleep.
The results were particularly unfavorable for people who slept less than five hours. In this group, 37 out of a total of 41 significant associations with increased risk were reported when compared with people sleeping between six and eight hours. Sleeping less than six hours is linked to a higher risk of heart failure, type 2 diabetes and chronic obstructive pulmonary disease, for example.
However, this doesn't mean that six hours is enough for everyone. The American Academy of Sleep Medicine and the Sleep Research Society recommend that adults regularly get at least seven hours of sleep, and the National Sleep Foundation's expert range is 7-9 hours for most adults and 7-8 hours for people over 65.
Sleep regularity matters too
It's not just the length that matters. Larger differences in sleep hours from one night to the next have been linked to a higher risk of anxiety and depressive disorders. Longer time awake after initially falling asleep has also been linked to several adverse health outcomes.
This supports the growing understanding of “healthy sleep“: that it is not just the hours in bed that matter, but also the continuity, regularity, and distribution of individual stages.
However, there are important limitations. Sleep was measured for only seven days, and stages were calculated using an algorithm and wrist movement, rather than polysomnography, which measures brain activity and remains the laboratory standard. SleepNet itself may underestimate REM and overestimate some stages of NREM sleep.
Most importantly, the study is observational. It does not prove that insufficient REM sleep causes dementia or that increasing deep sleep prevents diabetes. The reverse relationship is also possible: early, as yet undiagnosed diseases can change sleep years before diagnosis. After an additional analysis with a two-year period to limit this effect, about 60% of the initial results remained statistically significant.
The practical conclusion for now is simpler: regular and sufficiently long sleep remains a more important goal than trying to "increase" REM or deep sleep on our own. The data from the new study show why future medicine is likely to pay increasing attention not only to how many hours we sleep, but also to the quality and structure of those hours.