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Worrying about jet lag could actually make it worse, so try to relax on your next long-haul flight.

at Ludwig Maximilian University of Munich, Germany, and her colleagues have looked at potential psychological causes of jet leg 鈥� the temporary impact of a long flight in which someone feels out of sync with the new time zone. They may struggle to sleep, feel tired and have trouble concentrating.

鈥淚n biology when we think about a topic, we often think about what we can measure 鈥� like a molecule or something,鈥� Winnebeck says. 鈥淏ut a psychologist would think about the world very differently鈥� what you think about a disease can have a huge impact on it.鈥�

The team asked 90 people to keep a sleep diary for a week before and after taking a long-haul flight in 2018. Before the flight, they were also asked whether they expected to get jet lag and how bad they thought it would be. After the flight, they were asked to detail what their jet lag was like every day for a week using a questionnaire which quantified symptoms on a 60-point scale.

The researchers also took into account whether the participants were travelling east or west and how many time-zones they were planning to cross 鈥� six, on average.

The team found that, on average, jet lag lasted for around four days, but that it was less common than people thought it would be. More than 75 per cent of the participants said they expected to get jet lag, but only 54 per cent did.

The direction of travel and the number of time zones crossed had no notable effect on the extent of jet lag. 鈥淧eople are so variable,鈥� Winnebeck says. 鈥淟ength of travel could affect someone really badly, but have no effect on someone else.鈥�

The team did find that people travelling from west to east took longer to go to sleep in the week post-flight, though this group didn鈥檛 say that their jet lag felt worse compared with those who travelled in the other direction. Previous research has shown that this direction of travel makes your sleep worse because it is harder to fall asleep earlier than you鈥檙e used to, as opposed to staying awake for longer.

Instead, the researchers found that a person鈥檚 expectation for how bad their jet lag would be was the strongest predictor of how they said they felt. For every extra day someone expected their jet lag to last, the peak intensity of how bad they actually felt post-flight increased by a small amount. For example, someone who expected five days of jet lag reported a peak intensity nearly twice as high as someone expecting just one day.

This could be a form of 鈥�nocebo鈥� effect, a version of the placebo effect in which the expectation of harm can lead to a greater negative outcome, says Winnebeck.

鈥淚t could be a nocebo effect or it could be that the people who didn鈥檛 worry about jet lag just went about their normal lives after their flight 鈥� which helped the body get in sync quickly,鈥� Winnebeck says.

at the University of Oxford says it 鈥渕akes complete sense鈥� that expectation would play a role in how bad jet lag is. But he notes that human studies are extremely complicated and lots of factors may be at play beyond anxiety. 鈥淔ew people get good sleep on a long flight 鈥� how might that affect jet lag?鈥� he says.

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