This fits with another finding from the flight attendant study, which also discovered changes in the brain. Scans showed that the group with the busy schedule had smaller right temporal lobes on average, compared to than the other group. Perhaps it wasn't surprising that busy individuals also obtained lower scores on tests of memory.

There is a higher incidence of various types of cancer in airline staff who frequently cross multiple time zones

But the bad news for people who are frequently jet-lagged doesn't end there. Laboratory mice with induced jet lag show faster tumour growth. Foster speculates that if our circadian system is disrupted, it might stop us fighting off tumours at an early stage. On a more positive note, when a normal circadian rhythm was restored in the mice, tumour development slowed down.

Meanwhile in the lab chronic jet lag has been found to lead to a sequence of disease processes in mice that begins with non-alcoholic fatty liver disease, before progressing to hepatitis and a build-up of scar tissue in the liver, followed by a type of liver cancer.

Of course, these studies were conducted in mice, not people – so the specific findings may not apply to us. But there is a higher incidence of various types of cancer in airline staff who frequently cross multiple time zones.