Studies suggest that women who work at night over many years, such as flight attendants, are more prone to breast cancer. (CBC)

The Danish government has begun to compensate women who developed breast cancer after long spells of working night shifts.

Denmark's decision follows a ruling in 2007 by the International Agency for Research on Cancer or IARC, the cancer wing of the World Health Organization, that overnight shifts probably increase the risk of developing cancer, although other factors could also contribute.

The United Nations agency placed overnight work in the same category as anabolic steroids and ultraviolet radiation in terms of cancer risk.

Scientists suspect that overnight work could be dangerous, because it disrupts the circadian rhythm, the body's biological clock. The hormone melatonin, which can suppress tumor development, is normally produced at night.

For years, evidence has mounted on the health effects of overnight shifts, including disturbed sleep, fatigue, digestive problems and greater risk of accidents on the job.

Denmark's move compensates almost 40 women who worked overnight shifts, such as flight attendants who developed breast cancer after stints on the graveyard shift and moving across time zones, BBC News reported.

In 2006, Dr. David Blask, a cancer researcher at the Mary Imogene Bassett Hospital in New York told a conference in Ottawa that female night shift workers show a 50 per cent to 80 per cent higher risk of breast cancer.

Blask suggested light at night could be suppressing production of melatonin in female night shift workers.