Updated

A large study tracking more than 95,000 people found that time spent in different sleep stages correlates with risk for a wide range of diseases. People who spent more time in REM sleep had lower risks of 83 conditions, including dementia, heart failure, and Parkinson's disease, while more deep sleep correlated with lower risk of type 2 diabetes and major depression. The findings, drawn from long-term health tracking data, add to growing evidence that sleep architecture—not just sleep duration—may be an important, modifiable factor in long-term disease risk, though researchers caution the associations do not prove causation.