Researchers at the University of Bergen in Norway compared 1350 individuals who work 41 to 100 hours per week with 9092 workers who worked 40 hours per week or less using the Hospital Anxiety and Depression Scale (PDF).
They found that workers who worked more than 40 hours per week were 50 percent more likely to report symptoms of anxiety and depression, with the risk of depression increasing with the number of hours worked. These rates were particularly high for workers in strenuous manual labor jobs.
ABSTRACT: Working Overtime is Associated With Anxiety and Depression: The Hordaland Health Study.