10/04/2026
The goal was never just "eight hours." Inside those hours, the target is around 90 minutes of deep sleep, and that's the part almost nobody measures.
Start with total sleep. Large studies keep landing on about 7 hours as the point of lowest mortality risk, with risk rising on both sides, so longer isn't safer. A dose-response meta-analysis in the Journal of the American Heart Association found the lowest all-cause mortality at roughly 7 hours (Yin et al., 2017), and a UK Biobank analysis of more than 60,000 people found that sleep regularity, consistent bed and wake times, predicted mortality even better than duration did (Windred et al., 2024).
Inside that night, the quality target is around 90 minutes of deep sleep (stage 3, slow-wave sleep), roughly 13 to 23% of an average night. Think of it as the amount to protect and aim for, not a hard number you have to hit.
How to protect it. Keep your total around 7 hours and keep your timing consistent. Inside that, the main lever is body temperature, since you only drop into deep sleep as your core cools, so a cool room (about 65 to 68°F) and a hot bath 60 to 90 minutes before bed both help. Intense exercise earlier that day is one of the most reliable ways to increase deep sleep. And the most underrated factor is alcohol, which sedates you but suppresses deep sleep in the first half of the night, exactly when it should peak, which is why you can sleep 8 hours after drinking and still wake up unrepaired.
Two notes. Wearable "deep sleep" numbers are rough estimates and can be off by a lot, so watch the trend over weeks, not any single night. Deep sleep is one of the few things that measurably supports how your body repairs.
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Sources: Yin et al., Journal of the American Heart Association, 2017 (sleep duration and mortality); Windred et al., SLEEP, 2024 (sleep regularity and morta
