Let me count the light: Accounting for intensity, duration and timing of light when predicting sleep and subjective alertness in field studies

Throughout the day, we are exposed to light that varies drastically over time. Correct quantification of the light is important when predicting sleep and subjective alertness in the field, yet doing so, is a complex challenge. In the current manuscript, we explore the feasibility of a novel, practical method to quantify light exposure, based on the data collected in two field studies (in late spring and winter). Data include indicators of sleep, subjective alertness, and personal luminous exposure. We explored Time above Threshold (TaT) and Mean Light Timing above Threshold (MLiT) metrics, as well as their interaction, to quantify intensity, timing, and duration of light in testing not only circadian but also acute alerting effects of light in the field during office hours. For both measures, sensitivity analyses were performed across a large range of illuminance thresholds. The aim was to explore if these analyses would render indications for (a range of) effective thresholds, and to test if this alternative method of quantifying light would outperform simple averaging over specific time intervals. Despite the relatively small data set, the current approach seems promising particularly for predicting sleep: models performed slightly better than traditional models using average light exposure as predictor. More importantly, this method takes into account intensity, duration and timing, providing more detailed insights in the relation between luminous exposure and different outcome measures. We encourage this method to be explored further with larger data sets, discuss shortcomings of the current analyses and suggest potential directions for improvement