You're not imagining it: travelling really can mess with your period.

A few years ago, two close friends of mine travelled to Tokyo.

They had been there a few days when they went to a restaurant for dinner which advertised a ‘Ladies Special.’ Surely, they thought, it would simply be rude not to try a meal so clearly aimed at them, so they both ordered it.

That night, they returned to their hotel room to make a shocking discovery: they had both started their period, even though neither of them were due.

Upon returning home, they explained there had to be something freaky in the Ladies Special that triggered their menstrual cycles, but now there seems to be a far more logical explanation:

International travel messes with your period.

Travel can have a significant effect on both the timing and length of menstruation, and sometimes, your period can even disappear across time zones. 

Speaking to Broadly, researcher Anna Druet who works for the period-tracking app Clue, explained irregular exposure to light (which occurs when you travel across time zones) can throw off our circadian rhythms.

Circadian cycles last about 24 hours and regulate many of our physiological processes such as sleep, appetite and body temperature. But our circadian rhythms can also effect longer term processes like reproductive functioning.

When our rhythms don't match the environment, it triggers a domino effect (involving hormone production, for example) - which can impact not only the timing of your period, but ovulation too.

Druet says travel also tends to result in more painful periods, although researchers aren't quite sure why.

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So what about flight attendants, or those who are used to their circadian rhythms being out of sync with their environment? Surely their bodies just... adapt?

According to Druet, this isn't necessarily the case. Women who work shift work are at greater risk of endometriosis and have more painful periods, as well as higher rates of infertility.

So next time you're travelling internationally and find yourself wondering a) 'Where on earth has my period gone?' or b) 'Where the hell has my period come from?', remember something as simple as sunlight can have a huge impact on the way our bodies function.

Has travel ever impacted your period in an unexpected way?