Japanese researchers found that workers on rotating shifts were four times more likely to develop prostate cancer than those working regular hours, whether on day or night shifts.
Shift work has also been linked to an increased risk of breast and bowel cancers.
The study, which examined more than 14,000 workers over eight years, also found that night shift workers were at a slightly increased risk of prostate cancer. Eighty percent of the workers studied worked daytime hours, 7 percent worked nights, and about 13 percent rotated their work schedules from night to day.
The reason for the effects may be reduced secretion of the hormone melatonin in those who work irregular hours and, to a lesser extent, nights. Reduced secretion of melatonin has been linked to increased production of sex hormones, which help to regulate prostate tissues.