Women who work the night shift may have an increased risk of ovarian cancer, new research has warned.
The authors base their findings on 1,101 women with the most common type (epithelial) of advanced ovarian cancer; 389 with borderline disease; and a comparison group of 1,832 women without ovarian cancer.
The women, who were all aged between 35 and 74, were asked about the hours they worked, including whether they had ever worked night shifts.
Among the women with invasive cancer, around 1 in 4 (26.6%; 293) had ever worked nights, compared with 1 in 3 (32.4%; 126) of those with borderline disease and around 1 in 5 (22.5%; 412) 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 healthcare, food preparation and service, and office and admin support the most common types of employment.
Working night shifts was associated with a 24% increased risk of advanced cancer and a 49% increased risk of early stage disease compared with those who worked normal office hours.
A greater proportion (27%) of women who described themselves as "owls" had worked night shifts than women (20%) who were "larks"(morning types).
The risks of either advanced ovarian cancer were slightly higher (29%) among "larks" than among "owls" (14%), although difference this was not statistically significant. Findings were similar for borderline tumours — 57% and 43% for "larks" and "owls," respectively.
Only women aged 50 and above were significantly more likely to have ovarian cancer if they had worked nights.
One possible explanation could be linked to melatonin, a powerful hormone that is normally produced at night, but suppressed by ambient light, and which regulates reproductive hormones, particularly oestrogen.
Melatonin also scavenges harmful free radicals and boosts production of other antioxidants in the body.
The study has been published in Occupational and Environmental Medicine.