Analysis of data from the Whitehall II study found that people who worked 11 or more hours per day had an increased risk of developing depression.

Researchers collected data from 1,626 men and 497 women involved in this long-term study of British white-collar workers. None of the study participants had depression at baseline in 1991-1993. After an average follow-up of 5.8 years the participants were tested again for depression.

After controlling for several socioeconomic, life-style, and work-related factors, researchers found that people who worked 11 or more hours per day were between 2.3 and 2.5 times more likely to have developed depression than people who worked 7 to 8 hours a day.

The annual rate of depression among these participants was 3.1%, which is lower than the 5% seen in the general population. This could be due the fact that no one with preexisting psychological distress took part in this study. Limitations of this study include the small number of cases of depression and its observational nature, which means that some unmeasured factors may have affected the relationship between work hours and depression.

It is also not clear if these results apply to other occupational groups, such as blue-collar workers.