How much sleep do I really need?

Most adults do best with 7 to 8 hours of sleep per night. This is shown by an overview of 11 systematic reviews involving more than 4.4 million participants from 30 countries. The strength of the evidence varies by outcome, from low to high¹.

Consistently sleeping fewer than 7 hours is associated with a greater risk of obesity, diabetes, high blood pressure, cardiovascular disease, stroke, mental health problems and premature death. More than a third of American adults did not reach this minimum in 2014². These are observational findings, so cause and effect have not been established.

Sleeping too much also carries risks. In a cohort study of more than 322,000 participants followed for an average of 14 years, the risk of death at 10 or more hours per night was 34% higher in men and 48% higher in women compared with 7 hours. The same U-shaped pattern, in which both too little and too much sleep are associated with higher mortality, also applied to cardiovascular disease and cancer³.

Similar thresholds apply to brain health in older adults. Among older Chinese adults, sleeping fewer than 6 hours was associated with a 22% higher likelihood of mild cognitive decline. Sleeping more than 8 hours was associated with a 78% higher likelihood, compared with 6 to 8 hours. The most favourable point was around 7 hours. A short afternoon nap of less than 30 minutes was associated with a 27% lower likelihood of decline compared with not napping at all. This is a cross-sectional snapshot, so cause and effect are uncertain here as well⁴.

Do you exercise in the evening? Then pay attention to timing. Vigorous exercise within 4 hours of bedtime is associated with a later time falling asleep, shorter sleep and a higher nighttime heart rate. If you stop more than 4 hours before bedtime, or opt for light activity, this disadvantage does not apply⁵. A note of caution: this study was partly conducted by researchers affiliated with the company that supplied the measurement equipment.