Why Your Brain Is Literally Going to Get Bigger This Summer

But here’s when it will shrink back to normal.

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  • A new study finds your brain changes size from season to season.
  • The changes vary between brain portions and male versus female brains.
  • The data are from thousands of MRIs taken over 15 years at a neurology practice.

    Brace for a big brain summer: Your brain swells to its largest size in the warmest months, says a new study in PLoS ONE.

    Scientists from Hartford Hospital’s Olin Neuropsychiatry Research Center studied the brain scans of more than 3,000 healthy people over a 15-year period. The researchers found the sizes of different brain portions were consistently larger during the northern hemisphere’s warm summer months and smaller during the cold winter months.

    Hartford, Connecticut, which averages a high of 84 degrees Fahrenheit in July and a high of 36 degrees in January, is “an ideal location to test weather and seasonal effects because it is near sea-level, experiences four distinct seasons, and a wide range of weather conditions,” the scientists say. They continue:

    “Change in daylight is a significant factor in seasonal studies, but few studies have taken into account weather conditions, and no studies have examined the effects of weather on brain volume. Weather is often described as temperature, precipitation, and wind speed, but the most significant driver of weather is barometric pressure. Unlike temperature and humidity which are well-controlled in MRI scanning environments, pressure is ubiquitous and thus a good weather variable to explore.”

    But barometric pressure alone doesn’t affect your brain size—seasons also play a part. In the study, the brain scans showed subjects’ subcortical gray matter volume decreased between January and August and increased between August and December. The subjects’ left- and right-cerebellum cortex sizes, meanwhile, increased in volume between January and June and decreased between July and December.

    What’s going on here, anyway? While the scientists can’t say for sure, they do have a few theories.

    For one, they say your blood flow changes with the barometric pressure or ambient air temperature, which can also affect brain volume. Plus, your blood oxygen could be a factor. In a separate experiment, mice that were deprived of oxygen had higher blood flow in their “micro” brain vessels, but lower blood flow in larger “macro” vessels.

    As for what’s next with these statistical findings in hand, the results are a headache—literally. Chronic headache sufferers have a lot invested in seasonal and even daily swings in what affects the brain. The researchers say studying such a big sample size gives them a tool that will help inform future research.

    When scientists have tested the effects of weather and season on the human body in the past, they’ve found links between those factors and the incidence or severity of disease. Sales of headache medicines—and spontaneous recovery rates—also spike when barometric pressure drops, the researchers say.

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