What causes jet lag? © iStock

What causes jet lag?

Jet lag occurs when our internal body clock is out of sync with the time of day, causing tiredness and difficulty sleeping - but what causes it?

Asked by: Val McClean, via email

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Jet lag, or desynchronosis, occurs when our internal body clock is out of sync with the time of day, causing tiredness and difficulty sleeping. It’s commonly brought on by long-haul air travel, but it takes more than just a long journey to give you jet lag. “If you flew from England to South Africa, even though it’s a long flight, there’s no jet lag because there’s no time zone transition,” says biological rhythms expert Prof Jim Waterhouse. “The difference [with jet lag] is that it messes up the body clock.”

Animals, plants and bacteria all have an internal clock. In humans, this circadian clock is found at the base of the brain, in a structure known as the suprachiasmatic nucleus.

“What it does in humans is make us active in the daytime, and inactive and able to go to sleep at night,” says Waterhouse. “This clock gives you stability, so that if you do wake up in the middle of the night you’ve got a better chance of getting back to sleep.”

When we fly to a different time zone, the change is too dramatic for our clocks. “The body clock is slow to adjust, and that mismatching causes jet lag,” he says. Environmental cues like mealtimes and the light-dark cycles of the days help the clock to adapt, but how long it takes depends on how far you’ve travelled.

“It’s equivalent to about one or two time zones per day,” says Waterhouse. “If you fly to Los Angeles, which is eight time zones to the west of us, it’ll take you about four days to adjust.”

Jet lag is much worse flying east – forward in time, essentially – than when you fly west, and it’s because our body clocks don’t keep perfect time. “The body clock tends to run rather slow. It doesn’t run with a period of 24 hours, but rather with a period slightly more than that: about 24-and-a-half,” explains Waterhouse. “So it’s easier for the clock to delay because it naturally wishes to do so.”

Our slow-running body clocks are also the reason why over 90 per cent of travellers to Australia from the UK choose to stay up for 12 hours longer, rather than try to sleep 12 hours earlier. It’s still not clear why jet lag hits some people harder than others, but genetics, fitness and age have all been suggested.


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