Fig. 2: Light exposure profiles from two different individuals.

A Double plot of light exposure profiles averaged over five-minute bins for two individuals (blue vs yellow) over two consecutive days. B Quantification for each light exposure profile for the first 24 h. Black vertical line separates day 1 from day 2. Dotted lines show the recommended minimum light levels expressed in melanopic equivalent daylight illumination (melanopic EDI) during sleep (≤1 lx), 3 h before sleep (≤10 lx) and during day time (≥250 lx) according to Brown et al. ¹². The two light exposure profiles in (A) vary drastically and are associated with different behavioural subtypes (yellow vs blue profiles): while both individuals experience the same light onset in the morning around 06:30, individual 1 (blue) receives brighter light earlier in the day (first time above 250 lx mel EDI at 09:40) compared to individual 2 (yellow; at 13:05), spends more time above 250 lx (08:15 vs 04:14), receives brighter light on average (1635 lx vs 334 lx) and experiences the brightest hours earlier in the day (brightest 10 h midpoint at 14:45 vs 17:35). These differences could be due to job situations (e.g., shift work, lighting conditions at work, office vs outdoor job), individual preferences (e.g., sunseeker vs avoider), hobbies (indoor vs outdoor sports activities) or chronotype (Individual 1 is an intermediate chronotype while individual 2 is classified as late from the Munich Chronotype Questionnaire⁶; MCTQ). In the current example, subjective light exposure data revealed (data not shown) that individual 1 slept until 9:00, spent the morning indoors until 12:00 (daylight indoors), remained outdoors until 21:00 (daylight outdoors), received some electric light indoors until 22:00 and then spent the night sleeping without any reported light source. Individual 1 thus mostly reaches recommended light levels for pre-sleep and during sleep but could receive more bright light early in the morning. Individual 2 (yellow) experienced electric light exposure from midnight to 1:00, spent the night sleeping with light coming in through the window until 14:00, received daylight indoors until 18:00, spent 1 h outdoors in daylight until 19:00 and was inside with electric light until midnight. Individual 2 thus receives light above the recommended levels for pre-sleep and the sleep environment and receives too little light in the morning. Data based on a data collection in Tübingen, Germany, following the protocol described in Guidolin et al. ¹⁰⁹ and visualised with the R package LightLogR¹¹⁰.