Outdoor Circadian Rhythm Morning Light Calculator

How it works

Your body clock responds most strongly to bright light soon after waking. This calculator therefore sets your optimal window at 30 to 60 minutes after your usual wake time, which is the window named in the brief for this tool. If sunrise falls inside or after that window, the advice shifts to starting at sunrise instead, because light before sunrise is not available outdoors.

Recommended duration depends on brightness: 20 minutes in bright sun, and 40 minutes under overcast skies, where the light reaching your eyes is much weaker. These are practical planning figures, not medical prescriptions, and they assume you are outdoors with your eyes open, not looking directly at the sun.

When you choose the estimate option, sunrise is approximated from a base of 6:30 a.m. at the equinoxes, moved earlier in summer and later in winter, with a larger swing the further north the latitude band is. Tropical latitudes move the least. This is a rough planning estimate only; for an exact sunrise, type the real time from a local forecast.

How to use your result

Plan an outdoor activity inside the window: a walk, breakfast on a balcony, or simply standing outside. Keep sunglasses off for the session if it is safe and comfortable to do so, since tinted lenses cut the light your eyes receive. In the evening, dim bright lights about two hours before bed so the morning signal is not undone by late-night brightness.

Related guides

• Morning Sunlight and Your Body Clock
• Resetting Your Circadian Rhythm After Travel
• Sunrise Exposure for Better Sleep