In a study tracking astronaut health, 22 of 24 International Space Station (ISS) visitors suffered headaches almost three times as frequently as when on Earth. Even some astronauts with no history of headaches may experience migraine and tension-type headaches during stays of 10 days or longer in space, reported a new study published in Neurology.

The causes remain unclear, according to the study’s author, W. P. J. van Oosterhout, of Leiden University Medical Center in the Netherlands. “Changes in gravity caused by space flight affect the function of many parts of the body, including the brain,” he said in a press release.

Studying Space Headaches

The astronauts from the European Space Agency, the U.S. National Aeronautics and Space Administration (NASA) and the Japan Aerospace Exploration Agency underwent health screenings prior to their missions. They also completed questionnaires documenting their headache history. Once in space, they completed the questionnaires for the first seven days and weekly thereafter.

The contrasts were striking. Before their ISS trip, nine astronauts reported no headaches over the past year. Only three mentioned experiencing a headache that interfered with their lives during that period. None had a history of recurrent headaches or had been diagnosed with migraines.

The astronauts reported 378 headaches while in space. Of the total headaches, 170, or 90 percent, were tension-type and 19, or 10 percent, were migraine. Once back on Earth, none reported headaches the first three months after their return.

Read More: Why and How Do Astronauts Get Sick in Space?

Why are Astronauts Getting Headaches?

Van Oosterhout suspects that the part of the brain responsible for balance and posture receives different signals in microgravity. This confusion can lead to space motion sickness, of which headache is a frequent symptom.

However, drawing an explicit connection between weightlessness and headaches could prove difficult, according to van Oosterhout. The study’s relatively small number of participants means establishing cause and effect is statistically impossible. And the “self-reporting” of headaches is considered a less conclusive measurement than one not involving the astronauts’ opinions, instead of, say, a doctor’s independent measurements.

Still, the contrast between headache severity and frequency in space versus Earth is notable. Next steps include further investigation into both the cause of and treatment for “space headaches,” said van Oosterhout.

“Further research is needed to unravel the underlying causes of space headache and explore how such discoveries may provide insights into headaches occurring on Earth,” said Van Oosterhout.

Read More: Back Pain is Highly Common Among Astronauts

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