How many hours did you work last week?

If it was more than 39, you may be putting your mental health at risk. At least, that’s what a new study from Australia suggests.

The study, published in the journal Social Science & Medicine, found an “overall workhour-mental health threshold” of 39 hours for women, beyond which mental health declines.

For men, the threshold was 47 hours — not because they are mentally stronger, the study’s authors are quick to point out, but because they spend less time doing unpaid domestic work, including child care, in their homes.

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“Long work hours erode a person’s mental and physical health, because it leaves less time to eat well and look after themselves properly,” said Huong Dinh, the study’s lead author and a research fellow at Australian National University, in a released statement.

“Despite the fact that women on average are as skilled as men, women on average have lower paid jobs and less autonomy than men, and they spend much more time on care and domestic work,” she added. “Given the extra demands placed on women, it’s impossible for women to work long hours often expected by employers unless they compromise their health.”

Long hours

As background information in the study points out, work hours had been generally declining for many decades — until the 1980s came along. In 1980, 9 percent of American adults reported working 50 hours or more a week. Twenty years later, in 2000, 14 percent cited having a similar workweek.

Australia’s working-hour trends have followed a similar pattern.

The numbers are much grimmer, however, when only adults with full-time jobs are included in the data. According to a 2014 Gallup poll, full-time workers in the U.S. put in an average of 47 hours per week, and about 40 percent log at least 50 hours a week.

Indeed, 18 percent of the Americans polled said they spend a grueling 60 or more hours a week on the job.

“Some time spent in work generally improves mental health,” write Dinh and her colleagues. “But few studies have identified what the turning point might be.”

Reaching the limit

To help identify that point, the researchers analyzed data collected from more than 8,000 Australian adults, aged 24 to 64, who had participated in the Household, Income, and Labour Dynamics in Australia (HILDA) Study. The data included information on the participants’ mental health, wages and work hours, as well as other variables.

The analysis revealed that, on average, the maximum number of hours a week that the participants could work before their mental health began to suffer was 39, even after controlling for such factors as income, type of work and flexibility of work schedule.

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But that threshold number — 39 hours — “hides an important gender difference,” stress Dinh and her co-authors in an article they wrote on the study for The Conversation. “Women usually spend more time caregiving and have very different experiences on the job. This is because they have lower pay and less paid leave entitlement.”

That uneven playing field affects the workhour-mental health threshold, as the researchers explain:

When systematic differences in resources and rewards on and off the job are also taken into account, our study shows the work hour limit widens further to 34 hours for women compared to 47 hours for men. This gives men a 13 hour time advantage on the job, largely because they spend much less time on care or domestic work than women. Only if women were to spend very little time on care or domestic work, and if they had the same resources and rewards on and off the job, would the work hour limits converge. Under these assumptions men and women without care responsibilities can work up to 48 hours before their mental health is affected. However, anyone who spends significant time caring for others or doing domestic work is unable to work long hours without facing a likely health trade-off.

In line with other studies

The study, like all research, has several important limitations. It’s an observational study, so it can’t prove that the long hours were what contributed to people’s poorer mental health. Also, it included only Australians, so the findings may not be applicable to populations from other countries, including the U.S.

Yet the study’s findings are worrying, nevertheless, for they add to other research that has suggested long working hours are bad for mental health. For example, a 2012 British study involving civil servants reported an association between repeatedly working overtime and the risk of developing depression.

Other research has linked long working hours — including among white-collar workers — to poor physical health. One study found, for example, that once we pass a threshold of 45 working hours a week, we begin to raise our risk of developing cardiovascular disease, including high blood pressure, angina, coronary heart disease, heart attack and stroke.

Unfortunately, most Americans don’t have a choice about working fewer hours — or even about taking more time off. The U.S. remains the only industrialized country in the world that has no legally mandated paid annual leave (or parental leave), which is among the reasons why we’re ranked 30th out of 38 countries for “work-life balance.

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FMI: You can read the study on the Social Science & Medicine website.