Acute effect of light and time of day on thermal physiology, perception, and behavior.

Implementing novel, human-centric building control strategies that account for the interaction of multidomain factors, light, temperature, and time of day, can enhance occupant comfort and promote energy savings. However, their successful implementations require a robust framework that clearly explains the physiological mechanisms through which light exposure influences thermal perception and evaluation. To date, most studies have primarily relied on the hue-heat hypothesis, which attributes changes in thermal assessment solely to visual color associations, overlooking the physiological impact of light, particularly blue-enriched bright light, on the circadian rhythm and thermoregulation. In this study, we investigated the influence of light intensity on thermal physiology, perception, and behavior within a circadian context, employing skin temperature measurements, subjective questionnaires, and fan-use behavior as outcome measures. 20 healthy adults participated in four experimental sessions combining two illuminance levels (bright vs. dim, with identical spectral composition) at two times of day (07:00 and 14:00) under warm conditions, both steady-state and fan-induced dynamic ones. Results showed that skin temperature followed its natural circadian rhythm, being lower in the early morning than at midday. Notably, bright light exposure significantly suppressed the circadian rise in skin temperature in the morning, shifted thermal sensation votes from slightly warm towards neutral, and improved thermal comfort votes compared to dim light or midday exposure. This effect was evident under steady-state warm conditions and persisted following fan use. In contrast, under the fan-induced slightly cool condition, neither light intensity nor time of day significantly affected thermal assessment. These findings underscore the fundamental role of circadian physiology in thermal comfort and suggest that strategically timed light and thermal exposure can optimize comfort by aligning ambient conditions with the body's internal rhythms.