The latest report, published in Occupational and Environmental Medicine, focuses on ovarian cancer.
The finding of an increase in the risk of ovarian cancer with night shift work is consistent with those found for breast cancer, write researcher Parveen Bhatti, PhD, and colleagues from the Fred Hutchinson Cancer Research Center in Seattle.
Increase in Risk
The study included slightly more than 1,100 women with the most common type of advanced ovarian cancer and about 390 with early-stage disease. There was also a comparison group of more than 1,800 women without ovarian cancer.
The women (ages 35-74) were asked about the hours they worked, including whether they had ever worked night shifts.
Among the women with advanced cancer, around 1 in 4 had ever worked nights, compared with 1 in 3 of those with early-stage cancer, and around 1 in 5 of the comparison group.
The stint of night shifts averaged between 2.7 and 3.5 years across all three groups of women, with jobs in health care, food preparation and service, and office and admin support the most common types of employment.
The authors conclude that working night shifts was associated with a 24% increased risk of advanced ovarian cancer and a 49% increased risk of early-stage cancer.
The increased risk was restricted to women who were 50 and older.
'Larks' and 'Owls'
The new study also found that the risk of ovarian cancer among women who did night-shift work varied according to their preferred sleep schedule.
Women who said they preferred being awake in the morning ("larks") were at slightly higher risk. But when compared to women who preferred nighttime ("owls"), the difference was not significant.
Thomas Erren, MD, from the University of Cologne in Germany, writes in an editorial that the sleep dynamic makes sense.
"It seems intuitive that individuals who wake up very early in the morning (larks) and are most alert in the 8-16 hours that follow would also work in this time window with relative ease. Conversely, individuals who prefer to sleep in (owls) are most alert in late evening hours and can be expected to perform with greater ease in much later time windows," he writes.
The negative side to this comes when work does not fit in with a person's preferred sleep schedule, he continues. "Larks and owls experience more stress and strain when working outside their preferred time windows of activity."