In a study that became well-known in the 1990s, Canadian pilots were found to have higher than average rates of prostate cancer. More recently, a meta-analysis of several studies found that female flight attendants have higher than average rates of breast cancer and melanoma. Research in which flight attendants were asked about their health found the chances of having cancer (which could be of any kind), were higher than in the general population in women, but not in men.

However, these studies are observational, so, we can't be certain that it's chronic jet lag which is causing the increased cancer risk. Precise causes are hard to disentangle from other factors such as exposure to cosmic radiation while flying. If jet lag is to blame, the study of flight attendants' brains which saw differences depending on the staff's rotates, could suggest that careful scheduling and adequate rest times could mitigate these some of the risks for airline staff.

Easing the effects

Less frequent air travel has not been shown to bring these risks. But there are some things you can do to protect yourself from unpleasant symptoms, if you are crossing many time zones.

You can start before you even leave, by tilting your body clock in the right direction. If you are flying east, then go to bed one to two hours earlier than usual for a few days before your flight. If you are flying west, do the opposite, going to bed one to two hours later than usual.