What the Morning Light Consistency Calculator measures
Circadian rhythms are kept in step with the day mostly by light reaching the eyes, and by how regular your sleep and wake times are. This calculator turns three habits into a single 0 to 100 planning score: how much your wake time wanders from day to day, how many minutes of outdoor light you get after waking, and how much your bedtime drifts. A higher score means a steadier routine on paper. It is a habit-tracking aid, not a diagnostic tool or a measurement of your actual body clock.
It is most useful when you already keep a sleep log or wearable record. Take the standard deviation or typical spread of your wake and bed times over a couple of weeks, estimate your average outdoor minutes in the first hours of the day, and see which lever moves the score most. If you work shifts, have a medical sleep condition, or are treating insomnia, follow the advice of your clinician instead of a simple score.
The formula and its variables
Score = 70 − 0.45 × wake variance + 0.9 × light minutes − 0.35 × bedtime variance, then limited to the range 0 to 100.
- Wake variance (min): how far your wake time typically swings, in minutes. Each minute costs 0.45 points.
- Morning outdoor light (min): minutes outside soon after waking. Each minute adds 0.9 points.
- Bedtime variance (min): typical drift in when you go to bed. Each minute costs 0.35 points.
- Tracked days: used only for the projected score, which adds 2.5 × log base 2 of (days + 1), again capped at 100. Longer tracking earns a modest bonus.
- Light-to-variance ratio: light minutes divided by wake variance (at least 1). Instability points is simply 100 minus the score.
Worked example
With the default inputs (wake variance 35, light 22 minutes, bedtime variance 40, 14 tracked days): 70 − 15.75 = 54.25; plus 19.8 gives 74.05; minus 14 gives 60.05, which the calculator shows as 60.0/100. For the projected score, log base 2 of 15 is about 3.907, times 2.5 is 9.77, so 60.05 + 9.77 = 69.8/100. The ratio is 22 / 35 = 0.63 and instability points are 40.0. Now tighten wake variance to 20, light to 30 and bedtime variance to 15: 70 − 9 + 27 − 5.25 = 82.75, a jump of about 22 points, and most of it comes from the extra light and lower wake variance.
Common mistakes and how to interpret the score
- Entering the gap between your earliest and latest wake time rather than a typical spread will overstate variance and lower the score.
- Counting time behind a window as outdoor light. Indoor light is far dimmer than daylight, so log only time actually outside.
- Treating the score as a health rating. It is a weighted habit index with fixed coefficients, so use it to compare your own scenarios over time.
- Expecting the tracked-days bonus to matter much. It grows slowly, so consistency itself matters more than logging length.
Frequently asked questions
Is a score of 100 realistic? The formula caps at 100, and very high light minutes can reach it even with some variance, so treat anything near the top as "very steady routine" rather than perfect sleep.
Which input helps most? Per minute, morning light carries the largest weight (0.9), then wake variance (0.45) and bedtime drift (0.35). Cutting wake variance by 20 minutes is worth 9 points.
Should I include weekends? Yes. Weekend wake-time shifts are a common source of variance, so include them if you want the score to reflect your real week.
Does this replace medical advice? No. It is an educational planning model. Persistent sleep problems deserve a conversation with a healthcare professional.