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If you could know one number about your sleep that predicts your risk of dying early, developing heart disease, or losing cognitive sharpness as you age, it would be your total sleep time. Not how long you lie in bed. Not how rested you feel. The actual hours your body spent asleep, tracked objectively, night after night.
The relationship between sleep duration and health follows a striking pattern: both too little and too much sleep are linked to worse outcomes, with a sweet spot in the middle. Knowing your number, and watching how it trends over weeks and months, gives you something a subjective sense of "I slept okay" never can: a measurable signal you can act on.
Total sleep time (TST) is the sum of all minutes you spend genuinely asleep during a recording period, whether that is a single night or a full 24 hours including naps. A wearable sensor estimates this by tracking your wrist movement and heart rate patterns, then using an algorithm to classify each moment as sleep or wake. This is different from "time in bed," which includes the minutes you spend lying awake before falling asleep, waking up during the night, and lingering in bed after your last sleep cycle ends.
Wearable devices are reasonably good at detecting when you are asleep. Compared to polysomnography (PSG), the gold-standard overnight sleep study conducted in a lab, wrist sensors correctly identify sleep about 97% of the time. The catch is that they are much worse at detecting wake: they correctly classify wake only about 33% to 45% of the time. That means your wearable may slightly overestimate your total sleep time by counting some quiet wakefulness as sleep.
The largest study linking sleep duration to cardiovascular outcomes followed over 116,000 people across 21 countries. It found a clear U-shaped curve: people who slept 6 hours or fewer per day and those who slept more than 8 hours per day both had a higher risk of death and major cardiovascular events compared to those sleeping 6 to 8 hours. The increased risk was especially pronounced on the long end, where sleeping more than 8 hours was associated with roughly 1.3 to 1.4 times the risk of major cardiovascular events or death.
Multiple large meta-analyses confirm this pattern. A dose-response analysis pooling 40 prospective cohort studies found that 7 hours appeared to be the point of lowest risk for all-cause mortality, with risk climbing on both sides. In a Hispanic/Latino clinical cohort, both sleeping 6 hours or fewer and 9 hours or more were significantly linked to higher 10-year cardiovascular risk. In a UK Biobank study of over 407,000 people, long sleep (9 hours or more) was tied to higher risk of death and cardiovascular disease.
If your wearable consistently shows you are sleeping fewer than 6 hours or more than 9, that pattern deserves attention. It does not mean one bad night is dangerous. It means a sustained pattern at either extreme is associated with meaningfully higher risk.
Short sleep is linked to a cluster of metabolic problems. In a study of over 1,000 Kuwaiti adults, people sleeping less than 7 hours had higher BMI, blood pressure, triglycerides, cholesterol, insulin resistance markers, and CRP (a protein that signals body-wide inflammation). A meta-analysis of prospective studies confirmed that short sleep significantly raises the odds of developing type 2 diabetes, obesity, and hypertension.
For people who already have type 2 diabetes, the stakes are even higher. A study of nearly 19,000 adults with diabetes found that both short and long sleep durations independently increased the risk of new cardiovascular disease and death. Keeping sleep in the 7- to 8-hour range appears protective for blood sugar control and long-term outcomes.
A prospective study of over 7,300 middle-aged and older Chinese adults found that people sleeping 7 to 8 hours total per day had the lowest risk of cognitive impairment. Both shorter and longer sleep were associated with higher risk. Large changes in sleep duration over time, in either direction, were also harmful to cognitive function.
In a separate study of cognitively healthy older adults, shorter wearable-measured sleep time (assessed by actigraphy, a wrist-worn motion sensor similar to consumer trackers) predicted higher levels of tau protein in spinal fluid, a marker of the type of brain cell damage seen in Alzheimer's disease. This association was strongest in people carrying the APOE4 gene variant, which raises Alzheimer's risk. While this does not prove that short sleep causes dementia, it suggests that tracking your sleep duration over years could provide an early window into brain health.
A systematic review and meta-analysis found that both short and long sleep durations are significantly associated with an increased risk of chronic kidney disease. The pattern mirrors the U-shaped curve seen with cardiovascular disease and mortality, reinforcing that sleep duration is not a single-organ issue. It reflects system-wide biological maintenance.
"Normal" sleep varies by age, and there is no single magic number. The following recommendations come from the National Sleep Foundation's expert consensus panel and are supported by large population studies. Because wearable sensors may slightly overestimate sleep (by counting quiet wakefulness as sleep), your device-reported number could read 15 to 30 minutes higher than a lab-measured value.
| Age Group | Recommended Range | Context |
|---|---|---|
| Teenagers (14 to 17) | 8 to 10 hours | Sharp drop from childhood; many teens fall well short on school nights |
| Young adults (18 to 25) | 7 to 9 hours | Sleep debt from irregular schedules is common |
| Adults (26 to 64) | 7 to 9 hours | Population average is about 7 to 7.5 hours; roughly 1 in 4 adults sleeps less than recommended |
| Older adults (65+) | 7 to 8 hours | Sleep patterns shift with age; total time may be slightly shorter |
In pooled data from over 1.1 million people across three countries, adults reported a mean total sleep time of about 7.1 hours. Roughly 6.5% reported sleeping less than 6 hours, and about 1 in 4 slept less than the age-specific recommendation. These are self-reported numbers, which tend to overestimate true sleep by 30 to 70 minutes compared to objective measurement.
Total sleep time varies dramatically from night to night. Objective data from over 73 million recorded nights shows that people typically sleep 20 to 35 minutes longer on weekends than weekdays, and 15 to 20 minutes longer in winter than summer in the Northern Hemisphere. Add in the effects of stress, travel, caffeine, alcohol, and illness, and any single night's reading is a poor representation of your true sleep pattern.
This variability is not just noise. Higher night-to-night swings in sleep duration predict worse outcomes on their own, independent of average sleep time. In the Multi-Ethnic Study of Atherosclerosis (MESA), irregular sleep duration was linked to higher risk of cardiovascular events even after adjusting for how much people slept on average. In another study, people whose nightly sleep varied by more than 60 minutes had roughly 80% higher risk of becoming obese over follow-up.
The practical takeaway: wear your tracker every night for at least 7 to 14 consecutive nights before drawing any conclusions. Look at your weekly average and how much it bounces around, not any individual reading. Get a baseline over 2 weeks, then recheck after making any changes. For ongoing monitoring, review your monthly averages at least quarterly.
Several factors can make your wearable's total sleep time reading unrepresentative of your actual sleep health.
If you have been estimating your sleep based on when you got into bed and when your alarm went off, your number is probably wrong. In a study of 1,910 adults across four ethnic groups in the United States, self-reported sleep overestimated objectively measured sleep across all races, with differences averaging 30 to 70 minutes. In older Irish adults, the agreement between diary-reported and wearable-measured sleep was very low. People with insomnia symptoms showed the largest gaps between what they reported and what devices measured.
This matters because most of the large studies linking sleep duration to health outcomes relied on self-report, which means the true risk thresholds may be slightly different from what has been published. Using a wearable gives you a more accurate, objective measurement that you can track consistently.
If your 2-week average total sleep time is consistently below 6 hours or above 9 hours, or if your night-to-night variability is extreme (regularly swinging by more than 60 minutes), here is a practical path forward.
Evidence-backed interventions that affect your Total Sleep Time level
Total Sleep Time is best interpreted alongside these tests.