One study compared female flight attendants working on rotas that gave them little rest between flights, with flight attendants with more forgiving schedules who crossed fewer time zones and travelled less frequently. The staff in the group with the more demanding schedule were found to have higher levels of the stress hormone cortisol in their saliva.

Getty Images We are well aware of how jet lag can play havoc with sleeping patterns, but less aware of the health problems it can contribute to (Credit: Getty Images)Getty Images
We are well aware of how jet lag can play havoc with sleeping patterns, but less aware of the health problems it can contribute to (Credit: Getty Images)

This is fine in the short term, and we need cortisol to cope with the strain of waking up in the morning and to help us through stressful moments. But long-term exposure to high levels of cortisol can suppress the immune system and raise the risk of cardiovascular disease.

The impact of jet lag on the brain goes beyond just hormones. In a rather gruelling study conducted at the University of California Berkley in the US, hamsters were given a four-week sleep schedule which was the equivalent of them flying from New York to Paris twice a week. When the hamsters were "jet-lagged", they struggled with tasks they usually found easy.

What really surprised the researchers was how long the effects lasted – a month later the hamsters still weren't back on top form. Neuroscientific investigations revealed that while jet-lagged the hamsters had stopped creating as many new brain cells as usual in the hippocampus, a region of the brain associated with memory.