Normalized Amplitude of Blue Light Exposure (NA BLE) as a Novel Index for Circadian Light Hygiene: Associations with Actigraphy Measures and Seasonal Dependencies

Background — Light exposure (LE) critically regulates circadian rhythms, influencing sleep, mood, metabolism, and longevity. Quantifying circadian light hygiene is challenging due to high inter-individual variability. The Normalized Amplitude of Blue Light Exposure (NA BLE) metric provides a standardized assessment of BLE dynamics and range. We evaluated NA BLE in high-latitude Arctic residents, leveraging extreme seasonal light variations as a natural experiment to characterize its seasonal dependencies and associations with circadian parameters. Methods — Twenty-seven healthy Arctic adults underwent longitudinal assessment across seasons, spanning winter solstice, spring equinox, and summer solstice. Wrist actigraphy (ActTrust 2) captured physical activity (PA), wrist temperature (wT), light exposure (LE), and blue light exposure (BLE). Results — NA BLE exhibited clear seasonal variation, with a peak in spring and a nadir in winter, showing consistent patterns across sexes and populations (irrespective of indigeneity). Larger NA BLE correlated positively with indicators of circadian robustness, including Circadian Function Index (CFI) (p<0.0001), PA (p=0.001), and Inter-Daily Stability (p=0.008). Conversely, smaller NA BLE was associated with later circadian timing markers such as L5 onset of PA and BLE (p<0.001), bedtime (p=0.007), and lower stability of non-parametric actigraphy measures. Conclusions — Larger NA BLE significantly correlates with enhanced circadian robustness and healthier circadian timing, demonstrating its value despite seasonal fluctuations. NA BLE reliably measures circadian light hygiene, unaffected by sex or population subgroup. These results validate its utility in Arctic settings and advocate for its broader application in light exposure assessments to advance public health.