Background Nonstandard work schedules (NSWSs) have been linked to increased risk of depression, but the evidence is mostly from cross-sectional studies and data on the risk distribution across various groups is lacking. We examined the impact of different NSWSs on depressive symptoms and assessed effect modification by sex, ethnicity, and immigrant status.

Methods We used five waves of longitudinal data from the Understanding Society, the UK Household Longitudinal Study (UKHLS), which were collected biennially from 2010/11 to 2018/19. The study sample comprised 11371 employed workers aged 16 to 58 years in 2010/11. Generalised estimating equation with logistic regression was used to assess crude and adjusted effects of NSWS types (work time arrangement, work hours, and weekend working) on depressive symptoms. Independent correlation was specified in all the models as the correlation structure for the repeated exposure and outcome measures based on preliminary investigations using the quasilikelihood under the independence model criterion (QIC). To account for incomplete data during follow-up, we applied multiple imputation by chained equations and imputed 70 datasets. The regression analyses were repeated with the imputed dataset. Sampling weights were applied to account for the complex design of the UKHLS data.

Results After adjusting for confounders, only weekend work was associated with depressive symptoms. Workers who worked most/all (OR 1.24; 95%CI 1.13 – 1.37) or some (OR 1.10; 95%CI 1.02 – 1.18) weekends had a higher risk of depressive symptoms than workers who did not work on weekends. Ethnicity modified the association between work hours and depressive symptoms, indicating a higher risk among White workers working > 40 hours/week (OR 1.10; 95%CI 1.02 – 1.20) and Asian workers working <35 hours/week (OR 1.72; 95%CI 1.32 – 2.25) compared to White workers working 35-40 hours/week. No effect modification by sex or immigrant status was observed. Results from the complete-case and multiple imputation analyses were generally similar to each other.

Conclusion Weekend work, long work hours (for White workers), or working fewer than 35 hours/week (for Asian workers) were associated with increased risk of depressive symptoms. These findings indicate the need to review the working conditions of workers exposed to NSWSs, especially those working on weekends and Asian workers who work part-time.

Our study was based on longitudinal data with a long follow-up and a relatively large sample. However, we focused on the pre-pandemic era to avoid potential bias relating to the effects of COVID-19 on the labour market.