The study of more than 9000 women found those who worked more than 49 hours a week gained an average of about 1.9 per cent of their weight over a two-year period – or about 1.3 kilograms for a 69-kilogram woman. Those who worked part-time had an average weight gain of 1.5 per cent – or about 1 kilogram.
"These findings suggest that not working may have some protective effect against weight gain and may help promote weight loss," says the study, published in the International Journal of Obesity.
"This may be related to those women having more time to spend on maintaining a healthy body weight." The study, which examined women aged 45 to 50, found those who worked long hours, defined as 41 to 48 hours a week, or very long hours, more than 49 hours a week, were also far more likely to smoke, drink at unhealthy levels, sleep less and not exercise.
About 65 per cent of those who worked long hours drank at risky levels, compared with 42 per cent of those who were not in the workforce and 53 per cent who were unemployed.
The study said employed women may be more likely to gain weight because they have less time for exercise, sleeping and preparing home-cooked meals.
The lead author, Nicole Au, from Melbourne's Monash University, said the impact of long work hours was particularly evident among the women who gained the most weight.