Why do we get jet lag?

THAT'S THE WHY:If you travel long distances for work or holidays, you have probably experienced the crushing tiredness, irritability and general disorientation of jet lag.

From falling asleep at midday local time to lying wide awake and starving at 4am, it can play havoc with your internal schedule.

So what is it about crossing several time zones rapidly that throws our bodies into such disarray?

“Misalignment between endogenous rhythms and the light/dark cycle can result in circadian rhythm sleep disorders, including jet lag,” explains a 2009 review paper in the journal Travel Medicine and Infectious Disease.

And if that sentence alone threw you into disarray, let’s take a gulp of strong coffee and work through it.

Your body contains biological clocks, which send levels of particular proteins rising and falling over a period of about 24 hours. When such “circadian” rhythms are aligned, things can go pretty smoothly.

But where does that trip from Heathrow to Tokyo come in? One of the big factors in setting the “master” clock in your brain – which is thought to help regulate the local clocks in other parts of your body – is the amount of light hitting the back of your eyes, particularly natural daylight.

Crossing several time zones rapidly in one go disrupts your usual light/dark cycle and can affect these circadian rhythms. In other words, your internal clocks run amok.

They eventually realign and your bodily systems start to sing from the same hymn sheet again, but before they do you could feel out of step.