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Why International Travel Messes with Your Menstrual Cycle

Crossing time zones can send your period into hiding. But why? And where does it actually go? We asked a bona fide period expert.
Illustration by Ashley Goodall

It's not clear who the first woman to menstruate in space was, but the first American astronaut to take tampons into space was Sally Ride. In preparation for the mission on STS-7 in 1983, scientists fretted over what kind of tampons she should take with her, and how to prevent them from floating away, should the slippery suckers get loose (they ended up stringing them all together). Engineers also tried to guess how many she might need for a week-long mission in space. "Is 100 the right number?" they asked. "No," Sally Ride replied. "That would not be the right number."


The menstrual cycle is a bit like a relay race: one hormone triggers another, which triggers another.

Eventually, someone menstruated in zero-gravity and lived to report back that it was fine. But, back here on earth, travel does have a huge effect on the timing and length of the menstrual cycle. If you've ever taken an international flight and crossed several times zones, it's likely you've experienced your period literally disappearing into thin air, probably over the Atlantic or the Pacific.

But why? Where has it gone? Research scientist Anna Druet, who works at cycle-tracking app Clue, illuminates why our period can literally disappear over time zones.

BROADLY: Why does flying internationally and switching time zones mess up your menstrual cycle?
Anna Druet: There are many biological processes that function in cycles [Read: A lot of things in your body work in cycles]. "Circadian" cycles, which last about 24 hours and respond to external cues, impact your body temperature, sleep, hormonal production and appetite—among other things.

Our circadian cycles also impact our longer cycles of reproductive functioning, and visa versa. Exactly how the two interact is not fully understood, but if circadian rhythms become out of sync with the environment—via international travel, for example—reproductive hormones can also be affected.

The menstrual cycle is a bit like a relay race: one hormone triggers another, which triggers another, which triggers a physical response like ovulation or menstrual bleeding. If you don't produce as much estrogen in one cycle, for example, ovulation may be delayed, which can affect the timing of your fertile window, as well as your period.


When we travel across time zones, we suddenly become exposed to light at different times of the day. This above all throws off our circadian rhythms. Research has shown that even a small amount of dim, artificial light triggers hormonal changes in the body.

What does this translate to in our bodies?
If you go on an international flight and your circadian rhythm is thrown off, your cycle might come a little earlier than normal, or it might come a little bit later. It might be heavier or lighter, and it might last a little bit longer or a little bit shorter. It depends on when the travel happens during your cycle and if you've ovulated or not.

Consequently, this also affects your fertility window. Oh and this also usually calls for more painful periods, but we don't know why.

Is it easier on our bodies to travel one way versus another?
People tend to experience more jet lag when flying West-to-East—it's harder for your body to adjust to a shorter day than a longer day. So, it will likely take you more time to adapt when flying from New York to Berlin, than it will flying from Berlin to New York.

If you're someone like a flight attendant who travels all the time, does the body ever adjust? How does it know how to regulate itself without circadian rhythms to rely on?
It's tricky. People who are consistently working shift work have a higher risk of endometriosis and painful cramps. They also have higher rates of infertility, in that they might not ovulate as often and it might take them longer to get pregnant. There's also evidence of higher risk of breast cancer.


Even if you have consistent artificial light, it's not as strong as natural light. But even if you sat in a dark room all day, your biological rhythm and your circadian rhythms would still run—our brains have a kind of internal clock to mediate this.

But light acts as an entrainment [synchronizer] to our cycles, syncing our biological rhythms with our environments and communities. It's when these two things are out of sync that symptoms occur.

What about women in space? If they aren't exposed to the earth's circadian rhythms, what effect does it have on their cycles?
There isn't a lot of data on the effect of gravity on a woman's cycle. Changes to light exposure schedule in space may very well influence the menstrual cycle, but there hasn't been much research on this yet. Research on circadian rhythms in space have primarily focused on sleep.

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The impact of gravity menstruation is something that hasn't been researched much—one original concern was whether gravity changes would lead to retrograde menstruation, where menstrual blood flows inward toward the pelvic cavity. Research so far has shown that this is probably no more common in space than it is on earth—findings showed no reduction in menstrual flow, and no increase in related symptoms like abdominal cramping and shoulder pain.

But yes, women would still menstruate in space. Your body is still going through these ups and downs through hormonal cycling.


Speaking of space, are our cycles influenced by lunar phases, seeing as they are both 28-ish days long? Are our bodies connected to the moon's cycles?
No. There is no significant scientific evidence at all linking those things together. As far we know so far, it's a coincidence. Clue did the largest review of cycle data overlaid with lunar data and it did not find a correlation between the two.

Scientifically speaking, it's considered a coincidence.


But back to space. Most female astronauts opt to take the pill while in space and suppress their periods. However, for a 3-year-long mission, that means packing 1,100 pills—a hefty load to pack for space travel.

It turns out the final frontier in space is an IUD. As of now, it's yet to be tested in zero gravity.