Deep sleep, REM and what your watch can tell
Deep sleep and REM change with age and track long-term health. A watch reads them roughly, so Ageon looks at two weeks at a time and scores Apple Watch on its own scale.
A night of sleep cycles through stages every 90 minutes or so. Deep sleep comes mostly in the first half of the night. REM, when most dreaming happens, comes mostly in the second half. Apple Watch calls everything else Core.
Both change with age. A review of 65 lab studies found deep sleep falls by about 2% of the night every decade until around 60, then levels off. REM falls more slowly. So Ageon compares your share of each stage with what's typical for your age, not with a 25-year-old's.
Less of either has been linked to worse health later. In two US groups followed for years, every 5% less REM came with about 13% higher risk of death. In the Framingham Heart Study, each 1% of deep sleep lost per year came with 27% higher risk of dementia. Both are observational, so they show a link, not proof that more deep sleep or REM would change the outcome.
Your watch reads stages far less precisely than a lab. In a 2024 study against a full sleep lab, Apple Watch caught sleep versus wake well and got REM within minutes. It read about 50 minutes of deep sleep when the lab found 94, and counted the rest as Core. Rings and straps read deep sleep closer to the lab. That's why Ageon scores Apple Watch deep sleep against Apple Watch ranges, uses two weeks of nights instead of one, and gives stages less weight than how long and how regularly you sleep.
Time awake in the night counts a little too. More than 30 minutes a night is the usual research line for insomnia, though in UK Biobank it didn't predict mortality the way short or irregular sleep did.
Sleep with your watch on and at least 30% charged so it can track stages. Most of what raises deep sleep and REM is what raises sleep overall: enough hours, a steady schedule, no alcohol close to bed, and a cool, dark room.
Sources
- Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals. Ohayon et al., Sleep, 2004.
- Association of rapid eye movement sleep with mortality in middle-aged and older adults. Leary et al., JAMA Neurology, 2020.
- Association between slow-wave sleep loss and incident dementia. Himali et al., JAMA Neurology, 2023.
- Accuracy of three commercial wearable devices for sleep tracking in healthy adults. Robbins et al., Sensors, 2024.
- Associations between actigraphy-measured sleep duration, continuity, and timing with mortality in the UK Biobank. Saint-Maurice et al., Sleep, 2024.