Morning Light Is the Cheapest Circadian Intervention

Outdoor light in the first hours after waking shifts timing more than any indoor lamp.

The circadian system responds mainly to light intensity and timing. Outdoor daylight is orders of magnitude brighter than indoor lighting, even on an overcast day.

Laboratory studies show morning light advances the clock and evening light delays it, which is why the same lamp helps or harms depending on the hour.

Ten to thirty minutes outdoors soon after waking is the practical dose, and it costs nothing. Through a window is much weaker.

Evening dimming matters less than the marketing for tinted glasses suggests, but reducing bright overhead light in the last hour before bed is a reasonable, free adjustment.

Editorial note: this article summarises published research for general information. It is not medical advice. Discuss any intervention with a qualified clinician.