A large study involving nearly 100,000 people suggests that the quality of sleep, not just how long someone sleeps, may affect their risk of developing various diseases later in life. Researchers from Peking University in China analyzed data from the UK Biobank, which includes health information from a large number of people in the United Kingdom. The study used data collected from wrist accelerometers—devices that track movement and can estimate sleep patterns—worn by participants for seven days and nights. The researchers followed these individuals for an average of nine years and examined their health records for links to over 1,000 different diseases. The study found that people who experienced more REM (rapid eye movement) sleep—also known as the stage of sleep where most vivid dreams occur—had a lower risk of 83 different diseases, including heart failure, dementia, and Parkinson’s disease. On the other hand, greater amounts of deep sleep—also called slow-wave sleep, which is crucial for physical restoration—were linked to a lower risk of seven conditions, such as type 2 diabetes and clinical depression. However, participants who had irregular sleep patterns or who were more likely to wake up during the night were found to have a higher risk of conditions like anxiety and substance-use disorders. The study also found that people who regularly slept between six to eight hours per night had the lowest risk of several diseases. Those who slept less than five hours per night had a higher risk for 37 health conditions, while those who slept more than eight hours were also at increased risk for some diseases. The researchers noted that the optimal sleep duration for reducing disease risk was clearly within the six to eight-hour range for most of the conditions studied. The researchers emphasized that their findings are observational, meaning they can show associations but not prove that sleep patterns directly cause changes in disease risk. However, they suggested that aiming for a sleep duration of six to eight hours per night could be an effective way to reduce the risk of multiple diseases. These results offer new insights for health promotion and preventive strategies, highlighting the importance of sleep quality and regular sleep patterns in overall health.