HesapHanem

Light Exposure Timing Calculator

Enter your wake time and bedtime to see when to seek and avoid light.

How it is calculated

Enter both times in 24-hour HH:MM form (7:00 and 07:00 both work; 24:00, 11pm and 7.00 give no result). All three outputs are plain clock arithmetic on minutes, wrapping around midnight. The morning bright-light window is the hour that starts at your wake time: wake → wake + 60 min. The dim-light start is two hours before bedtime: bedtime − 120 min. The body-temperature minimum is estimated as wake − 120 min, following the rule of thumb that it falls about two hours before habitual waking. Bedtime affects only the dim-light start. If wake time and bedtime are the same clock time the tool shows no result, since that is not a day. It does not check that your sleep length is realistic and cannot know your personal rhythm.

Worked example

You usually wake at 07:00 and go to bed at 23:00. The morning bright-light window is the hour starting at wake time, so it runs 07:00–08:00. Dimming begins two hours before bedtime: 23:00 − 2 h = 21:00. The estimated body-temperature minimum is two hours before waking: 07:00 − 2 h = 05:00. So the plan is bright light from 07:00 to 08:00, dim lights from 21:00, and bed at 23:00, with the estimated low point at 05:00 falling two hours before you wake.

Try this example →

Common mistakes

  • Typing times with am or pm, or in a form like 7.00. Only HH:MM in 24-hour format is read, so 11pm gives no result and must be entered as 23:00, and midnight is 00:00, not 24:00.
  • Entering the time you wish you woke up rather than your habitual wake time. The temperature minimum and the light window both move with the wake time you type.
  • Expecting the bedtime to change the morning window or the temperature minimum. It only sets the start of the dim-light period.
  • Entering the time you turn the lights off while still awake in bed. The dimming hour is counted back from the bedtime you enter, so use the time you try to sleep.
  • Treating the temperature minimum as measured. It is a fixed two-hour offset from waking, and shift workers or very short sleepers may have a different true minimum.

Sources

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 places your estimated CBTmin two hours before your wake time and lays out a personal light schedule: a one-hour morning window starting at wake time, where seeking bright light (sunlight or a light-therapy box) helps anchor an earlier rhythm, and an evening window starting two hours before your bedtime, 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.

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