In a surprising discovery, researchers from the University of California, Santa Barbara, found that men’s brains shrink as the day progresses, shrinking by evening and then resetting overnight. This change appears to be linked to fluctuations in hormones such as testosterone, cortisol and estradiol.
The study, published in The Journal of Neuroscience, involved scanning a 26-year-old man’s brain 40 times over a 30-day period. The results showed that by 8pm each day, the overall brain volume had decreased, particularly in areas of the cortex that handle sensory input and visual information. The brain would then recover its size during sleep.
Researchers found that steroid hormone levels peaked in the morning and dropped by night. “Testosterone, estradiol and cortisol decreased from morning to evening by 61, 38, and 92%, respectively,” the study stated. This decrease in hormones was paralleled by the brain volume reduction.
Laura Pritschet, one of the study’s co-authors, explained that while women’s hormone levels fluctuate on a monthly cycle, men experience these shifts within a single day. However, co-author Elle Murata noted that researchers are not yet sure whether the hormones are the direct cause of the brain’s volume changes.
The findings highlight how brain regions responsible for memory and emotions—such as the gray matter—undergo daily cycles of growth and shrinkage. This may affect men’s cognitive abilities throughout the day, although more research is needed to understand the full implications.
Though this study focused on one individual, it challenges the assumption that hormone-related brain changes are exclusive to women. The research team hopes to expand their study to investigate how other factors like stress and sleep patterns impact brain structure.
