Introduction Higher winter depression prevalence at temperate latitudes has been attributed to low daylight levels. Daytime light exposure varies across jobs.

Objectives In a nationwide study, we examined occupational daytime light exposure levels and risk of initiating antidepressant treatment among men and women.

Methods We conducted a register-based follow-up study of all gainfully employed 526 839 men and 506 092 women in Denmark (56° N) born 1980 or later. We estimated annual mean occupational daytime (6:00–17:59 h) white light exposure levels (lux) for each year of employment 2000–2015 with a quantitative job exposure matrix (JEM). We followed participants from 2001 or first year of employment (if later) until first antidepressant treatment ≤ 2016 as recorded in a national prescription database. We calculated incidence rate ratios adjusted for register information on age, calendar year, family history of antidepressant treatment, and educational level. Smoking was assessed with a lifestyle JEM.

Results During follow up, the population accumulated 6 804 361 person-years and 30 816 men (5.8%) and 52 817 women (10.4%) initiated antidepressant treatment corresponding to 8 and 16 cases per 1000 person-years, respectively. Two percent of women and eight percent of men were exposed to an annual average above 500 lux. Among women, we observed reduced risk of initiating antidepressant treatment for those exposed the previous year to 500–999 lux (incidence rate ratio [IRR] = 0.92, 585 cases, 95% CI 0.85–1.00) and ≥1000 lux (IRR = 0.90, 294 cases, 95% CI 0.80–1.01) when compared with <250 lux. No association was apparent for men.

Conclusion Our results indicate a preventive effect of daytime light exposure on depression and other mental disorders treated with antidepressants for the small fraction of workers exposed to an annual mean above 500 lux at work, but only among women. Findings need replication in other countries.