Women who work shifts are more likely to retire early due to ill health than male counterparts, a study suggests.
Women who work nights may come home to a day of domestic duties
The health risks of shifts are already well-documented, but this Danish study points to a clear gender division.
Some 8,000 men and women working both regular and irregular hours were followed by Danish researchers.
Shift-working women were over a third more likely to claim an early pension for poor health, the Occupational and Environmental Medicine study found.
Women - whether shift or day workers - on the whole were more likely to claim the Danish disability pension than men, the team at the National Research Centre for the Working Environment in Copenhagen found.
But while male shift workers were no more likely to retire early than other workers, female shift workers claimed disability pension much more frequently than both their male counterparts and women who worked days.
The differences remained even after researchers had taken into account other risk factors, such as smoking, weight and socio-economic group.
"Shift work" in this study was defined as anyone whose hours regularly changed or who worked fixed evening and night work. These were compared with permanent day workers.
Studies have already linked shift work to a range of health conditions, including heart disease, breast cancer and peptic ulcers, as well as potentially causing problems during pregnancy.
Stress, lack of sleep and hormonal disturbances have all been cited as factors.
"It is therefore not surprising if the incidence of disability requirement is higher among shift workers, but we have no knowledge about why women should be more vulnerable to shift work than men as out study suggests," said report author, Dr Finn Tuchsen.
However occupational health experts speculated it could well be linked to the "double burden" that female workers carry.
"Women are much more likely to come home to domestic work, and the strain of this combined with what we already know about the impact of shift work could explain the findings," said Dr Phil Tucker of Swansea University.
Richard Jones of the Institution of Occupational Safety and Health said it was disappointing that the study did not differentiate between types of shift work and further probe the reasons for women's vulnerability.
"We need further research to look into what could be behind this, what difference length of time spent doing shifts makes, and if certain patterns are more problematic than others - then we can think about how to minimise the risk."
There is growing awareness of the health risks associated with shift work, as more and more are asked to adopt such patterns as a result of the 24 hour economy.
According to the most recent figures from the Health and Safety Executive, some 14% of the UK working population - or 3.6m people - now do shift work.