Working long hours appears to substantially increase a person’s risk of becoming depressed, regardless of how stressful the actual work is, a new study suggests.

The study, which followed 2,123 British civil servants for six years, found that workers who put in an average of at least 11 hours per day at the office had roughly two and a half times higher odds of developing depression than their colleagues who clocked out after seven or eight hours.

The link between long workdays and depression persisted even after the researchers took into account factors such as job strain, the level of support in the workplace, alcohol use, smoking, and chronic physical diseases.

Although the findings are “consistent with previous studies, the degree of increased risk was surprising,” says Bryan Bruno, MD, chair of the psychiatry department at Lenox Hill Hospital, in New York City, who was not involved in the research. “The biggest condition that I work with is depression, and it is often related to work stressors.”

Overworked junior and mid-level employees appear to be more prone to depression than people higher up the food chain, the study suggests. The length of the workday didn’t have a perceptible impact on the mental health of higher-paid, top-level employees such as cabinet secretaries, directors, team leaders, and policy managers.

That’s likely due to the amount of control higher-ups have over their own work, says Alan Gelenberg, M.D., who, as the chair of the psychiatry department at Pennsylvania State University, in University Park, is a higher-up himself.

“We have more control over what we work on; we can choose the fun stuff,” says Gelenberg, who was not involved in the study. “I do mostly what I want to do, and when I put in an extra hard week, it’s my choice.”

For those lower on the totem pole, the researchers say long hours at the office could contribute to depression in several ways—by creating family or relationship conflicts, for instance, or by elevating levels of the stress hormone cortisol.

Job insecurity and sleep deprivation also may help explain the increased risk of depression, Bruno says, noting that previous research has shown that poor sleep is a key ingredient in work-related depression. “I often really focus on that symptom,” he says, referring to his own patients.

Recent studies on overtime and depression have reported similar results, but most used a less rigorous standard for measuring depression. The new study included face-to-face consultations and used the American Psychiatric Association’s official criteria for clinical depression, making it one “one of the rare studies” to do so, says lead author Marianna Virtanen, Ph.D., a researcher at the Finnish Institute of Occupational Health, in Helsinki.

The new research, which appears in the journal PLoS ONE, drew upon data from a long-running study, known as Whitehall II, that includes employees from 20 London-based branches of the British civil service.

The study participants Virtanen and her colleagues focused on were all deemed mentally healthy when they were first evaluated between 1991 and 1993. Six years later, slightly more than 3% of the employees were found to have experienced clinical depression within the previous year. The more overtime they worked, the more likely they were to be depressed.

The majority of the study participants (52%) worked a normal seven- or eight-hour workday. Thirty-seven percent averaged nine- or 10-hour days, and 11% worked 11 hours or more.

The study had several shortcomings. The authors looked only at white-collar civil servants, for example, so it’s not clear whether the results would apply to blue-collar workers or to employees in the private sector.

Another question the study doesn’t answer is how long an employee can maintain 11-hour workdays before the risk of depression begins to rise: One month? One year?

“When we talk about working short increments of overtime versus long-term [overtime], we don’t know what’s too long,” Gelenberg says. “When do things tip over?”