The research team led by Dr. George Ebers of Oxford University, examined Canadian data on multiple sclerosis sufferers and found that the gender ratio has been rising for at least 50 years.

Canada has one of the highest rates in the world, along with many nations in northern Europe, prompting speculation genetics and geography may increase the risk for people who live in countries far from the equator.

More than 1,000 new cases of the disease will be diagnosed this year in Canada, and an estimated 55,000 to 75,000 people are currently living with the disease.

The researchers found that among people born in the 1930s, the ratio of women to men contracting the disease was 2:1, but by 1980 the numbers had risen, with more than three women with MS for every man.

Ebers, a professor of neurology at Oxford University suspects something in the environment may be contributing to the increased incidence of MS in women.

Other experts suggest that a vitamin D deficiency due to low sun exposure may be a contributing factor in the development of the disease.

The cause of MS is basically unknown, but most researchers believe it is an autoimmune disease; it usually appears between 20 and 40, but MS has also been diagnosed in children.

Other possible factors could be the changing role of women in the work force, dietary habits, increase in smoking among women, use of oral contraceptives, and changes in the timing of childbearing years.

Multiple sclerosis is unpredictable and caused by inflammation and patchy destruction of the protective myelin covering the central nervous system.

The disease attacks the brain and spinal cord leading to paralysis and sometimes blindness.

Multiple sclerosis initially may cause numbness and tingling in the extremities and temporary loss of vision in one eye.

As the disease develops, sufferers may experience a range of symptoms, including extreme fatigue, balance and co-ordination problems, muscle stiffness and weakness, as well as speech and cognitive difficulties.

The study is published in the November edition of the Lancet's neurology journal.

The researchers also found that chronic stress, measured by daily corticosterone levels, did not increase in the old mice experiencing the light-cycle shifts.

They say the underlying cause of the increased mortality is not yet clear, but could involve sleep deprivation or immune-system disruption.

Scientists believe the body's physiological reaction to time change may be a complex one and previous research has indicated that circadian clocks govern physiological rhythms in a great variety of tissues in the body, and that different aspects of the physiological clock can adjust to schedule changes at different rates.

The researchers speculate that the internal lack of synchrony among different physiological oscillations may have serious health consequences that are exacerbated in aged animals.

Experts say as there is an increasing amount of cross-time zone travel, and of a '24-hour' society, more research, on animals and humans, is needed in order that the risk can be properly assessed.

The research is published in the November 7th issue of the journal Current Biology.