In an announcement to be published Saturday in the journal Lancet Oncology, the International Agency for Research on Cancer, part of the World Health Organization, will label shift work as a “probable cause” of cancer.
The designation — rooted in the theory that the disruption of circadian rhythms could be a culprit — puts shift work on a par with ultraviolet radiation or anabolic steroids as suspected carcinogens, but does not say it is a definitive cause of cancer, such as cigarette smoking.
A random schedule of shifts – working a couple of days a week at night, a couple of days during the day – is the worst.
A phase-advancing shift-rotation (a week at night, followed by a week in the afternoon, followed by a week in the morning) is just as bad.
A slower, phase-delaying rotation (a month in the morning, followed by a month in the afternoon, followed by a month at night) is a little bit better.
Staying constantly on the night shift is almost as bad, mainly as it is almost impossible to keep it consistent, i.e., completely avoiding light during the day and exposing oneself to bright light during the night even on one’s days off. Social events and weekends produce a state of permanent jet-lag nonetheless.
But as in everything biological, there is a variation in population – some people are affected much more strongly than others. Let workers self-select their schedules as they can best feel on their own how the night work is affecting their physical and mental health.