The findings, based on a 2001 study of 40,000 Taiwanese aged 15 and older, were published on Tuesday in the Journal of Epidemiology and Community Health.

Overall, the study found that women are up to twice as likely to be insomniacs as men. Sleeplessness was also found more common among people who were oder, divorced or separated, had less education, poor health and earned less.

The study suggested long working hours were to blame for insomnia in men: Those with higher positions, usually men in Taiwan, had correspondingly greater education, and were required to work more hours.

* FROM NOV. 7, 1998: The mystery of sleep

Divorced or separated women appeared to have the highest levels of insomnia. Researchers suggested the stress associated with single parenthood, loss of income, or the stigma of a marriage breakdown could be possible factors.

Other factors that could cause greater insomnia in women were menstrual cycles including menopause, night-day body temperature fluctuations, depression and anxiety, the researchers wrote.

More studies needed

But even after accounting for these factors, the researchers still found that they could not fully explain why women get less sleep.

"The sex discrepancy in insomnia narrowed slightly after taking social role factors into consideration but was not explained by socioeconomic status," the authors wrote. "The persistent sex gap in insomnia warrants further investigation."

The researchers cited other studies that have shown that women get less good sleep than men in most cultures around the world. But they said this could not be totally explained by child rearing and other domestic responsibilities.

Children impact sleep

Students, non-smokers and regular exercisers tended to have better sleep quality, while the jobless who were actively seeking work had the highest level of sleep disturbances, the authors wrote.

The more children in a household caused higher levels of insomnia; but this factor affected women's and men's sleep equally, the study found.

The insomnia was assessed using criteria developed by the World Health Organization, and scored on a scale of 1 to 5.