Circadian disruption beneath the surface: a screening approach to identify actigraphy-based markers of split shift work in submariners

Identifying behavioral markers of disease risk associated with circadian disruption remains challenging due to reproducibility concerns, as human studies are often under-powered to detect the effects of interest. Submarines provide an extreme example of cir-cadian disruption, depriving individuals of natural light-dark cycles and limiting oppor-tunities for physical activity. To address these challenges, we developed an effect size-based method to robustly screen potential actigraphy-derived markers of the submarine environment. Actigraphy data was collected across the pre-mission, mission and post-mission periods. During the mission, crew members followed a 6-hours-on/6-hours-off split shift schedule, except for submariners with on-call duty or extended hours. Forty-one actigraphy variables were screened for their ability to differentiate between study periods and shifts. A Monte Carlo permutation test was used for this initial screening, followed by receiver operating characteristic (ROC) curve analysis for pairwise compar-isons. Light-related features, the slopes of regression lines fitted to the daily predicted dim light melatonin onset (DLMO) trajectories, and Pearson correlation coefficients be-tween each recording day and its corresponding predicted DLMO demonstrated the great-est potential to distinguish between study periods. Most high-performing features dis-criminated between split shift work schedules and on-call or extended-hours schedules. Only the relative amplitude (RA) and the average light intensity during the five least-illuminated hours of the day (L5) reliably distinguished between the shift performing the bulk of nighttime work and its counterpart. Although submarine settings offer an oppor-tunity to study a healthy population living under confined movement and dim light, de-tecting biological signals through actigraphy in this environment is inherently challeng-ing. Demonstrating robust effects under these conditions supports the use of actigraphy in dimly-lit, isolated, confined, and extreme (DICE) environments, but also its applica-bility to study other operational or clinical populations with reduced activity levels.