Work in shifts that have you getting up in the dark one week, going to bed after dusk the next?
You might want to consider finding a new job – a new study has linked disrupted sleep and frequent jet-lag to weight gain and increased likelihood of developing cancer.
Long-term studies in humans are difficult to conduct, so researchers in the Netherlands turned to mice with a genetic predisposition towards cancer, comparable to the BRCA gene carried by movie star Angelina Jolie.
Normally it would take 50 weeks for such mice to develop breast cancer, but in conditions that simulated constantly changing day/night schedules, onset came eight weeks earlier.
They also gained about 20 percent more weight than mice in the control group.
"The conclusion is that chronic changes in light schedules are a driving factor for breast cancer development, weight gain, and other metabolic problems," says senior study author Bert van der Horst of the Erasmus University Medical Centre.
"This outcome is in line with what has been found in epidemiological studies in humans, and now provides strong causal evidence for circadian disruption as a carcinogenic factor."
Past studies in humans have been criticised because of the sheer number of outside factors – for example, women working nights also tend to have higher rates of smoking and drinking, exercise less and don't eat as healthily – which can all contribute to weight gain and cancer. Mice don't have any of these associated problems.
The study also showed how neither a lack of vitamin D (from minimal exposure to sunlight) and melatonin suppression (caused by exposure to bright light, particularly blue) contributed to the problems, suggesting they're driven by disruption to the mice's circadian rhythms.
"The mechanism of the circadian clock as well as carcinogenesis is comparable in mice and man, and this model opens new avenues for further exploring how circadian disruptions affect the body and how you can intervene to reduce adverse effects," says Prof van der Horst.
The study, published today in journal Current Biology, urges people with genetic predispositions to cancer to avoid working in shifts or in trans-continental aviation.