Light Exposure Timing Calculator

About this calculator

Your circadian clock resets itself daily using light as its main cue, and how light shifts your rhythm depends entirely on when you see it relative to your core body temperature minimum (CBTmin), which typically occurs a couple of hours before your habitual wake time. Bright light seen after CBTmin — in the morning — advances your clock, pushing your sleep and wake times earlier over following days; bright light seen before CBTmin — late evening and night — delays your clock, pushing sleep later. This relationship is mapped by what chronobiologists call the phase response curve to light.

This calculator uses your wake time and bedtime to estimate your CBTmin and lay out a personal light schedule: a morning window where seeking bright light (sunlight or a light-therapy box) helps anchor an earlier rhythm, and an evening window where dimming lights and avoiding screens protects your natural melatonin rise instead of delaying it.

People adjusting to a new work shift, recovering from jet lag, or simply trying to fix a drifting sleep schedule use light-timing guidance like this because, unlike melatonin, light is the strongest signal the circadian clock responds to.