Track blue light exposure timing.
Light dose at the eye depends on brightness, duration and distance, and distance matters most because illuminance falls with its square — a phone at 25 cm delivers several times the dose of the same screen at 60 cm. Night modes cut short-wavelength output by roughly half. Combining those into an index gives a comparable score and an estimated delay in melatonin onset, which is the mechanism by which evening screens push bedtime later.
Evening light dose index
Dose index = screen hours x brightness/100 x (45 / distance in cm)^2 x 0.45 if a blue filter is active; score = dose index x 28
These figures are estimates based on population-average formulas and are intended for training and recreational planning only. They are not medical advice. Consult a qualified health professional before starting or changing an exercise programme, particularly if you have an existing medical condition.
Short-wavelength light suppresses melatonin and shifts circadian timing. The effect size depends on intensity and duration — a bright phone held close is far more disruptive than a distant TV.
They reduce short-wavelength output measurably, but they do not remove the alerting effect of engaging content. Dimming the screen usually helps more than shifting its colour.
As far as comfortable. Because dose falls with the square of distance, moving from 25 cm to 50 cm cuts the light reaching your eye by roughly three quarters.