A major new study shows that the composition of nightly sleep, not just its duration, correlates with the risk of developing a wide range of diseases later in life.

Researchers led by Jingsong Luo of Peking University analyzed wrist-worn accelerometer data from 95,559 UK Biobank participants who wore the devices for seven consecutive days and nights. The team used a deep-learning algorithm to estimate time spent in REM sleep, deep sleep, and light sleep, along with total sleep duration, sleep irregularity, and wakefulness after sleep onset.

Participants were followed for a median of 8.9 years, during which researchers examined links to more than 1,000 health outcomes using hospital records. Greater REM sleep was associated with lower risks for 83 diseases, including heart failure, dementia, Parkinson’s disease, Alzheimer’s disease, and multiple sclerosis. Greater deep sleep correlated with lower risks for seven conditions, such as type 2 diabetes, major depressive disorder, and sleep apnea.

Total sleep duration showed a nonlinear relationship with disease risk. Individuals who averaged six to eight hours per night had the lowest risks for 69 conditions. Those sleeping less than five hours faced elevated risks for 37 conditions.

Increased sleep irregularity and more time awake after falling asleep were each linked to higher risks for several conditions, including anxiety and substance-use disorders. The study, published September 17 in PLOS Medicine, was observational and cannot establish causation.

The findings reinforce longstanding advice on sleep duration while highlighting the potential protective role of specific sleep stages.