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Objective Depression imposes a heavy burden on the health of young adults. Disturbance of the circadian system is a key characteristic of depression. Bright light exposure during the day is the main stimulus synchronizing the circadian rhythm with that of the daily solar cycle. US and European workers spend more than 90% of their time indoor. We aimed to investigate whether outdoor levels of daytime light exposure at work prevent the onset of depression in young adult men and women.Material and Methods We conducted a cohort study of all Danish workers in the DOC-X cohort aged 19-31 years (315,381 men and 304,639 women) from January 1, 2004 through December 31, 2016 (total follow up 2,663,008 person-years). Diagnoses of depression were based on hospital contacts and prescriptions for antidepressants provided by the government health care system. Annual average daytime light exposure (lux) at work was assessed from participants’ work histories and a quantitative job exposure matrix (JEM) developed from wearable dosimeter recordings. Incidence rate ratios of depression were estimated using Poisson regression adjusting for age, family history, education, and JEM estimates of smoking and physical activity at work.Results During follow-up, 6,441 men (2.0%) and 12,270 women (4.0%) were diagnosed with depression, corresponding to incidence rates of 4.76 and 9.77 per 1,000 person-years. Workers exposed during the previous year ≥250 lux, levels typically seen in outdoor workers, showed adjusted incidence rate ratios for depression of 0.86 (95% CI, 0.81-0.91) for men and 0.92 (95% CI, 0.87-0.97) for women, compared with workers exposed <250 lux, levels typically seen in indoor workers.Conclusion Workers exposed to outdoor-equivalent daytime light levels had a decreased risk of depression. Integrating outdoor-equivalent daytime light exposure into workplace design and policy may prevent the onset of depression and benefit the mental well-being of many young adult workers.