Your total sleep score may be missing part of the picture

Hours asleep are the number most sleep apps put front and centre. But a large UK Biobank analysis suggests those hours are not all interchangeable. Researchers used wrist monitors to follow more than 95,000 people for a median of 8.9 years and compared estimated sleep patterns with later diagnoses.

More REM sleep—the stage most associated with vivid dreaming—was associated with lower risk across 83 later disease outcomes. More deep sleep was associated with lower risk across seven. Short total sleep mattered too: sleeping under five hours was associated with higher risk of 37 conditions, while many outcomes had their lowest observed risk in the six-to-eight-hour range.

That is a fascinating map of associations, not a prescription for chasing a particular REM score. Wrist monitors estimate sleep stages less precisely than a sleep laboratory, and the direction of travel can run both ways: underlying health problems may alter sleep. Still, the study is a useful nudge away from treating one nightly total as the whole story.