Objectives Nightshift work has been associated with cancer among men, but the mechanism underlying this association is not clear. We investigated whether male nightshift workers demonstrated changes in the normal circadian levels and secretion patterns of melatonin, cortisol and sex hormones that may be directly related to cancer risk.

Methods Participants were 185 male nightshift workers (NSW) and 158 male dayshift workers (DSW) employed as healthcare providers, aged 22–55. Urine samples were collected throughout work and sleep periods and assayed for various hormone metabolites.

Results Compared to DSW during their nighttime sleep, NSW had significantly lower levels of 6-sulfatoxymelatonin during daytime sleep, nighttime work, and nighttime sleep on their off-nights (57%, 62% and 40% lower, respectively). Urinary cortisol levels in the NSW were 16% higher during daytime sleep and 13% lower during nighttime sleep on off-nights, compared to DSW during nighttime sleep. While cortisol levels between NSW during night work and DSW during night sleep were not significantly different, metabolites of cortisol (e.g. cortisone, tetrahydrocortisol) were significantly increased among NSW. No significant differences were observed in testosterone or dihydrotestosterone levels between nightshift workers during their day sleep or night sleep compared to dayshift workers during nighttime sleep.

Conclusions Male sex hormones have been implicated in prostate carcinogenesis, however, results of this study indicate that the impact of nightshift work on cancer risk may occur through other mechanisms. Substantially reduced 6-sulfatoxymelatonin levels during night work, daytime sleep and even night sleep on off-nights among night shift workers were observed, and given the oncostatic properties of melatonin, this chronic reduction in melatonin among nightshift workers may represent an important carcinogenic mechanism. Corticosteroid secretion and metabolism was also found to be impacted by night shift work, which could have implications for cancer risk through its effects on immune function.