0028 Chronotype and Circadian Timing in Day and Chronic Night-shift Workers

Abstract Introduction In populations who live and work typical day-shift work hours, chronotype (i.e., morning lark or night owl) is tightly associated with dim-light melatonin onset (DLMO) timing, the gold standard marker of circadian phase. The association between these markers is less clear in populations chronically working fixed, overnight shift work schedules which may be vital for making population-based recommendations. We therefore examined the relationship between chronotype and DLMO in chronic, fixed overnight shift working nurses compared to fixed day-shift working nurses. Methods Sixteen volunteers (15 female; aged average ± SD 31.8±7.7yr) working either a fixed day (n=8) or nightshift schedule (n=8; 19:00-07:00 shift; duration of night-shift work 8.6 ± 7.2yr) participated in an ~8h evening in-laboratory stay in dim-lighting ( 5 lux). Beginning at 17:00, saliva was collected every 30 minutes to calculate circadian phase via salivary DLMO (3pg/ml threshold). Chronotype was determined via daily sleep/wake diaries collected over 7-days and defined as mid-sleep time on free days corrected for sleep loss on participant-identified work days (MSFsc). Pearson Correlations were used to determine associations between DLMO and MSFsc within each work-schedule group and circadian timing between groups was compared using Independent T-Test. DLMO was not captured in two night-shift working participants, thus they were not included in DLMO analysis. Results The timing of MSFsc and DLMO were significantly correlated such that later mid-sleep timing was correlated with later DLMO timing in day-shift (r=0.77, p 0.025), but not night-shift (r=0.005, p=0.99) workers. Moreover, the average MSFsc (01:25± 0:45 vs 11:16±01:35; p 0.001) and DLMO timing (19:13±01:04 vs 21:12±01:01; p=0.005) of the day-shift nurses were significantly earlier than the night-shift nurses. Conclusion Working and living on a day-shift schedule results in an association between chronotype and circadian timing, while a stable night-shift schedule results in no discernable relationship between the measures, despite a later DLMO timing. Understanding the connection between these sleep and circadian measurements could have implications for interventions to improve sleep, health, and performance in shift-working populations. Support (if any) OHSU School of Nursing PhD Student Scholarship Award, T32HL083808, K01HL146992