Physiologically-relevant light exposure and light behaviour in Switzerland and Malaysia.

backgroundLight synchronises the internal clock with the external light-dark cycle. Keeping this alignment benefits health and prevents diseases. Quantifying light exposure is, therefore, vital for effective prevention.

objectiveSince light exposure depends on photoperiod, culture, and behaviour, we investigated objective light exposure and individual light-related behaviour in Switzerland and Malaysia.

methodsIn this observational field study, participants (N = 39) wore a calibrated melanopic light logger at chest level for 30 consecutive days. At baseline and study end, the Pittsburgh Sleep Quality Index was assessed, and every 3 to 4 days, the Light Exposure Behaviour Assessment (LEBA) was filled.

resultsOur pre-registered analyses reveal that participants in Switzerland experienced brighter days (+3.16 times the average mel EDI) and spent more time (x1.9 times the duration) in daylight levels per hour of daylight, had ~1.5 h later bright light exposure in the afternoon, and stayed over 1 h longer in dim light conditions before bedtime. LEBA scores did not differ between Malaysia and Switzerland, and LEBA items were stable over time. LEBA items also correlated with objective light exposure variables in Switzerland but not Malaysia, with a medium effect size (range of absolute r = 0.32-0.48). SIGNIFICANCE: Our results highlight cultural and geographical differences in light exposure. We showed that subjective assessment of light-exposure behaviour can be related to actual light exposure and is ecologically informative, but this varies by culture. IMPACT: Light is a key environmental factor shaping human health, yet real-world exposure varies widely across geography and culture. In this study, we compared physiologically-relevant light exposure and self-reported light-related behaviour between Switzerland and Malaysia over a 30 day period using wearable sensors and questionnaires. We found that participants in Switzerland experienced brighter days and darker evenings than those in Malaysia, even after accounting for differences in photoperiod length. These differences reflect how climate, culture, and behaviour influence light exposure patterns. Our findings highlight the need to consider cultural and environmental context when developing recommendations and interventions to promote healthy light exposure in everyday life.