O026 Within-subject variation in circadian timing in shift workers repeating the same shift patternn

Abstract Introduction While inter-individual variation in circadian response to shift work has been demonstrated, the within-individual variation across repeated roster patterns is unknown. This study aims to examine the consistency in circadian timing during repeated occasions of the same shift pattern. Methods In an ongoing study, shift workers (n=24, age 36.79±8.83 years, 8 females) from multiple industries were monitored during two replications of their usual shift patterns, separated by at least 6 weeks. Shifts included consecutive night shifts, consecutive early morning shifts, and early-morning-to-night rotations. Participants completed daily sleep diaries and wore an activity monitor to measure sleep, and a light sensor to measure light exposure. Circadian phase was assessed at the end of each shift pattern via 6-sulphatoxymelatonin (aMT6s) acrophase (peak) in urine. Results Preliminary analyses (n=15) showed considerable within-subject variability in aMT6s acrophase at the end of each shift patterns. Mean difference in acrophase was 2.06±1.36 h (range 0.15-5.22 h). Individuals working consecutive night shifts demonstrate larger variation in acrophase (3.43±1.58 h; range 1.50-5.22 h), compared to those working consecutive morning (2.01±0.69 h, range 1.18-2.86 h) or early morning-to-night shifts (1.30±0.95 h, range 0.15-2.95 h). The mediating role of individual light exposure patterns in the variation in aMT6s acrophase will be presented. Discussion Preliminary findings highlight inconsistency in circadian phase within individual shift workers, possibly associated with variation in socially-driven behaviours. Further factors influencing variability in circadian timing, i.e., individual light exposure, will be examined, to support prediction of circadian timing for informing personalised interventions for shift workers.