Is a 2% Daylight Factor Sufficient for Robust and Timely Circadian Photoentrainment?

We spend over 90% of our lives indoors, often under suboptimal lighting conditions that can disrupt circadian photoentrainment and contribute to various health issues. This study investigates whether the widely accepted 2% average daylight factor (DF), mandated by building regulatory authorities, is sufficient for timely and robust circadian photoentrainment. Conducted as a field study, our experiment assessed the impact of a daylight exposure intervention in north-facing residential units on key circadian and cognitive markers, including dim light melatonin onset (DLMO), sleep parameters, subjective sleepiness, alertness, and vigilance. Our findings indicate that even a 6.7% DF, significantly above the mandated minimum, failed to consistently provide adequate light levels for optimal circadian photoentrainment. Reducing the average DF to 2% was associated with a trend toward later DLMO, although the aggregate difference was not statistically significant. The observed pattern of insufficient morning light exposure and elevated evening light exposure was consistent with conditions that can contribute to circadian phase delay. These results highlight the need for revised daylighting standards that account for the nonvisual effects of light and better support occupants’ health and well-being within the built environment.