Circadian rhythm
Sleep timing and consistency matter as much as duration
At a glance
| Study type | Systematic review |
|---|---|
| Year | 2020 |
| Journal | Applied Physiology, Nutrition, and Metabolism |
| Sample | n = 41 |
| Population | General adult |
Background
A 2020 systematic review of 41 studies (over 92,000 participants in 14 countries) examining sleep timing and sleep regularity. Later sleep timing and greater day-to-day variability were associated with adverse health outcomes; social jetlag was specifically harmful.
What is 'social jetlag'?
The mismatch between your weekday and weekend sleep schedules. If you wake at 6 AM Monday–Friday but sleep until 10 AM Saturday and Sunday, your body is experiencing a 4-hour eastward time-zone shift twice a week. The review found social jetlag associated with adverse cardiometabolic and mood outcomes.
Is consistency more important than duration?
It's not either/or. Both matter, and they're partially independent. The review found people with stable but suboptimal duration outperformed people with variable duration on several health endpoints — meaning a consistent 6.5 hours can beat a chaotic 8.
How was 'consistency' measured?
Studies used several measures: sleep regularity index (SRI), standard deviation of bedtime, midpoint-of-sleep variance, and weekday/weekend differences. The review treated these as related but distinct constructs and found the directional signal was consistent across measures.
What does this look like in pregnancy and postpartum?
Pregnancy and postpartum demolish baseline sleep regularity — third-trimester nocturia and postpartum infant feeding both fragment timing. The actionable read of this paper is to protect a fixed wake time as the anchor, since wake time has the largest downstream control over the next night's onset.
How does Solas turn this into a recommendation?
Solas tracks the variance of your bedtime and wake time across a rolling window and surfaces a 'consistency score.' When variance is high, the recommendation defaults to anchoring wake time, not bedtime.
Source
Chaput JP, Dutil C, Featherstone R, et al.
(2020).
Sleep timing, sleep consistency, and health in adults: a systematic review Applied Physiology, Nutrition, and Metabolism.
DOI: 10.1139/apnm-2020-0032
Read the original paper →
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