0024 Circadian Phase Response to Shift Work: The Role of Light

Abstract Introduction Shift workers exhibit substantial individual variability in circadian response to work rosters . Light exposure has been shown to explain a considerable proportion of this variability during night shifts. This study aims to explore the predictive role of light exposure for circadian timing across diverse roster patterns. Methods Thirty-eight shift workers (37.16±9.06 years, 18 females) were monitored during a usual 6-day roster. Roster patterns included consecutive night shifts, consecutive early morning shifts, early morning-to-night rotations and night-to-early morning rotations. Sleep-wake timing was assessed using daily sleep diaries and wrist actigraphy. Circadian phase was assessed on the first and last day of the roster via 6-sulphatoxymelatonin (aMT6s) acrophase (peak) in urine. Light exposure was continuously measured via a lapel-worn light sensor. The difference in light intensity between the main phase advance (3-7h after acrophase) and phase delay zones (3-7h before acrophase) of the light circadian phase response curve was calculated across 6 days. Phase advance/delay zones were adjusted daily to account for individual phase shifts. Results Substantial individual variability was observed in the magnitude and direction of phase shifts across roster patterns (mean magnitude of shift = 1.76 ± 1.45 h; range = -6.57 to +3.21 h). Following consecutive night shifts, aMT6s phase shifts varied from -5.13 h to +1.17 h (mean = -1.81 ± 1.74 h, 10 delay, 1 advance). Following consecutive morning shifts, aMT6s phase shifts varied from -5.41 h to +3.21 h (mean = +0.70 ± 1.82 h, 7 advance, 2 delay). The difference in light exposure between the advance and delay zones across 6 days significantly predicted the degree of phase shift in either direction (β = 0.001, p = .01, R² = .16). Including diurnal preference improved the model to explain 31% of the variance (adjusted R² = .31). Conclusion Findings highlight that individual variability in circadian phase responses to various roster patterns was partly explained by differences in light exposure during the critical periods of the light phase response curve. This research supports the prediction of circadian timing to inform personalised interventions for shift workers. Support (if any) National Health and Medical Research Council (NHMRC) (APP 2001234).