Outdoor night light exposure and sleep structure, a panel study using consumer-grade wearables

Background: Artificial light at night (ALAN) poses a growing public health concern due to its potential to disrupt circadian rhythms and affect sleep quality. This study examines the association between outdoor nighttime light (NTL) exposure and sleep duration using objective wearable-device data from a Chinese panel. Methods: Sleep metrics were collected from 4,690 participants (2017–2019) via Zepp Health smart bracelets. Outdoor NTL exposure was obtained from NASA's Black Marble product and categorized into tertiles. Mixed-effects regression models were employed to analyze the associations between NTL exposure and objectively measured sleep parameters, adjusting for demographic, behavioral, and environmental covariates. Results: The results suggest that NTL exposure (one standard deviation of 5.20 W·cm²·SR⁻¹) was associated with a 1.22-min longer light sleep duration (95% CI: 0.04, 2.41), whereas the overall linear association with the total and deep sleep duration was not evident. Subgroup analyses revealed stronger associations in elderly and lower BMI participants, with positive associations in non-drinkers, smokers, and participants from non-middle-income areas. Additionally, restricted cubic spline analyses suggested a non-linear pattern for deep sleep, with a modest decline at higher NTL levels. Conclusion: This study suggests that moderate NTL levels are associated with slightly longer light sleep durations, whereas higher NTL levels may negatively impact deep sleep. The findings stress the need for urban lighting guidelines that balance safety with sleep health, minimizing negative effects on circadian rhythms. This research also contributes to future studies on personalized light exposure, considering indoor factors and individual sleep behaviors.