Marie Tolver Nielsen

Objectives Night work disrupts sleep, which may increase inflammatory processes, pain sensitivity and pain perception. We investigated effects of night shifts on musculoskeletal pain (MSP) and other mental and somatic symptoms using day-to-day measurements in a within-person design. We also disentangled the effect of night work from the effects of shift-related differences in job demands and sleep. Methods We included 522 hospital employees with data on ≥1 day shift (n=1926) and ≥1 night shift (n=1422). Participants reported their working hours, MSP, other mental and somatic symptoms, sleep and psychosocial and physical job demands daily for 14 days. We estimated prevalence ratios (PRs) of MSP and symptoms when working night shifts compared with day shifts. Results After adjusting for age and job demands, night shifts were associated with pain in the lower extremities (PR 1.12; 95% CI 1.01 to 1.25), gastrointestinal symptoms (PR 2.12; 95% CI 1.26 to 3.53) and other mental and somatic symptoms (PR 2.54; 95% CI 1.31 to 4.91). Adjustment for sleep duration did not alter these findings. However, longer shift duration may explain the higher occurrence of lower extremity pain during night shifts. Conclusions We found no evidence of a general effect of night work on MSP. Only an acute effect on lower extremity pain was observed, which may be attributable to longer night shift duration. Employees also more frequently reported other types of symptoms when working night shifts, even after adjusting for job demands and sleep duration. Thus, physiological effects related to circadian misalignment may be a plausible explanation.

Night work and circadian disruption are linked to major public health challenges, e.g. cancer, cardiometabolic disease, and accidents. We established the 1001 nights-cohort to explore mechanisms underlying health effects of night work and circadian disruption. 1075 female hospital employees participated from September 2022 to April 2024. The data collection included a questionnaire, a blood sample, anthropometric measures, and sleep actigraphy and sleep diaries across 14 days. In subsamples, light exposure, physical activity, skin temperature, and blood glucose were measured continuously for 7 days, and saliva samples were collected five times across one day. The cohort consists of 2- and 3-shift workers with night work (66%), permanent night workers (7%), permanent evening workers or 2-shift workers without night work (9%), and permanent day workers (18%). Data comprise 4553 day shifts, 997 evening shifts, 1963 night shifts, and 6458 days without work. The poorest health was observed among permanent night workers and the group of shift workers without night work. The 1001 nights-cohort is the most comprehensive data within night work and working hour research due to the combination of questionnaires, biomarkers, technical measurements, and possibilities for linkage to historical and future register-based information about working hours from the Danish Working Hour Database (DAD) and diagnoses. With its repeated measurements within the same individual, the cohort will advance research on physiological and behavioral mechanisms underlying health effects of working hours, night work, and circadian disruption and deliver important scientific input for updating guidelines on healthy scheduling of working hours.