What you've heard
Ten minutes of morning sunlight sets your body clock for the day
Verdict
Right direction, invented number
Right direction, invented number. Get outside early, and stay longer when it is gray.
Does ten minutes of morning sunlight set your body clock?
Right direction, invented number. Get outside early, and stay longer when it is gray.
No study has tested ten minutes. The clock's response is steep at first and then flattens: in a laboratory study of evening light, twelve minutes of bright light produced about two-fifths of the shift that four hours did. Those volunteers had been kept in dim rooms beforehand, which makes the clock more sensitive, so the figures show the shape of the response, not a morning dose. Past moderate brightness, making light last longer helped more than making it brighter, and outdoor light, even in shade or under cloud, is many times brighter than a lit room.11
Where the book tests it
What the book recommends instead
Get outdoor daylight early in your day, roughly 10 to 30 minutes
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.How: within an hour of waking, step outside, not behind glass: walk part of the commute, coffee on the step, the dog. Longer when gray or shaded; open sky beats a courtyard; a lunchtime walk outside counts too. Sunglasses are fine. Why: morning light pulls the clock earlier, and outdoors is many times brighter than indoors.
Sources
- 11.Chang, A.M., Santhi, N., St Hilaire, M., Gronfier, C., et al. (2012). "Human responses to bright light of different durations." Journal of Physiology, 590(13), 3103-3112. https://doi.org/10.1113/jphysiol.2011.226555 (Evening light after a dim-light baseline; 12 minutes gave a 1.07-hour delay against 2.65 hours for 4 hours.) ; Dewan, K., Benloucif, S., Reid, K., Wolfe, L.F., et al. (2011). "Light-induced changes of the circadian clock of humans: increasing duration is more effective than increasing light intensity." Sleep, 34(5), 593-599. https://doi.org/10.1093/sleep/34.5.593
Note numbers are those of Chapter 7, Light and Dark.
This verdict is one of the "What you've heard" boxes in Chapter 7, Light and Dark, where the book tests the claims people repeat against the studies behind them.
Educational content, not medical advice. Disclaimer.