Timing

What It Is

The clock time you go to bed.

Why It Matters

Later bedtimes track higher mortality and cardiovascular risk across a string of large population studies, though the evidence is observational and agrees with itself less than it first appears. The studies that asked people what time they went to bed find the association; the one that timed sleep onset with a wrist monitor lost it once sleep length was taken into account. No study has identified a threshold either. Midnight is simply where researchers tend to cut the data, so treat it as a rough marker rather than a cliff edge.

Two things this is not. It is not your chronotype: being a night owl by disposition carries little independent risk, and the large cohort analyses find that ordinary risk factors, blood pressure, smoking, activity and the rest, account for most of the apparent link between evening preference and heart disease. It is not the whole curve either, because these studies also show higher risk among the earliest sleepers, but these are cohorts of older adults, and at that age an early bedtime may mark illness as much as cause it, the same reverse causation that makes long sleep look bad.

The proposed mechanism is circadian, and sleeping against your body clock appears to disturb glucose handling even when time in bed is unchanged, though that work is small and mixed. The more dependable effect is simpler: against a fixed wake time, a later bedtime just costs you hours. So treat bedtime as the end of the night you can actually move, and when sleep runs short, pull it earlier rather than pushing your wake time back.

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