The Bathroom Trip at 3 a.m.: How Bright, and in What Colour, a Night Light Can Be Before Its Melanopic Dose Rivals the Evening Screen

Night lights are bought by brightness and placed by convenience, while the quantity the circadian system counts is melanopic dose — spectrum-weighted light multiplied by time. This paper computes that dose for a stated four-minute bathroom trip under three common fittings, against the dose from a stated evening of screen use, 40 lux for two hours, which totals 2400.0 melanopic lux-minutes. A low amber fitting delivers 2.2 melanopic lux-minutes, 0.09 percent of the evening's dose; a bright daylight-white fitting delivers 114.0, 4.8 percent of the evening dose in four minutes. Inverting the calculation gives a design bound: to keep a single trip under two percent of the evening dose, light at the eye must stay below 12.0 melanopic lux — 66.7 photopic lux if amber, but only 12.6 lux if daylight-white. Spectrum, not brightness, is the variable that decides which side of the bound a fitting falls on. This study is simulated, not measured.