0768 Driving During the Biological Night: DLMO-Indexed Circadian Phase Predicts Driving Performance in Night Shift Workers

Abstract Introduction Night shift workers (NSWs) face an increased health burden compared to their diurnal counterparts, likely due to misalignment between an individual’s endogenous circadian rhythm and external work/real-world demands. This misalignment also negatively impacts performance on complex tasks; importantly, impacting one’s driving ability, perhaps beyond fatigue alone. Given the US’s heavy reliance on personal transportation over public infrastructure, NSWs face heightened dangers while chronically driving during one’s biological night-rest period. The current study examines prospective driving simulator data from NSWs to test whether circadian phase and fatigue impact driving performance. Methods NSW participants (N = 74, 51.4% Black, 83.8% female, aged 40.3 ± 12.1yrs) were recruited from Metro Detroit. Measures included: dim light melatonin onset (DLMO) derived via hourly saliva samples; phase angle of driving time relative to DLMO; sleepiness measured by the Karolinska Sleepiness Scale (KSS) at drive time, and driving performance as assessed on the York Driving Simulator using standard deviation of lane position (SDLP) and driving speed variability (Speed SD). The protocol included two 40-min simulated local-area freeway drives (3am, 7am) each during two study visits, totaling 259 drives. Mixed-effects regression models tested associations between circadian phase and fatigue on driving performance, adjusting for demographic characteristics and practice effects. Results Median DLMO among NSWs was 11:28 PM (range = 2:38 AM [Day1] – 6:58 PM [Day2]; SD = 7.1hrs). During the 3:00AM and 7:00AM drives, 58.4% and 63.5% of participants, respectively, drove during their biological night. Mixed-effects models revealed a significant quadratic effect of phase angle with both SDLP (β = 0.15, p = .006) and Speed SD (β = 0.16, p = .01), indicating worse driving performance with greater circadian misalignment. Conclusion NSWs conduct much of their work during their biological night, when circadian physiology promotes sleep and reduces alertness. Driving performance deteriorated most during the early-morning commute, reflecting the influence of circadian phase. This provides evidence that circadian misalignment can have dire consequences for driving performance. Support (if any) This work was funded by the NHLBI (K23HL138166, PI: Cheng).