Men have a daily hormone cycle — and it's synced to their brains shrinking from morning to night

The daily ebb and flow of hormones in the male body may play a role in shrinking the brain throughout the day, a study hints. After losing volume between morning and evening, the brain resets overnight, starting the cycle again, the research shows.

The study involved scanning a 26-year-old's brain 40 times in 30 days. Each magnetic resonance imaging (MRI) scan was collected at either 7 a.m. or 8 p.m., which is when levels of steroid hormones — namely, testosterone, cortisol and estradiol — are at their highest and lowest, respectively.

"Males show this 70% decrease from morning to night in steroid hormones," said study co-author Laura Pritschet, who's now a postdoctoral scholar in psychiatry at the University of Pennsylvania Perelman School of Medicine. The degree of change between morning and night narrows with age, but that general pattern persists throughout life.

"You can think of it almost like a pulsating rhythm from morning to night," Pritschet told Live Science. Females also experience a daily flux in hormones, but it's not as pronounced, she noted, because the menstrual cycle is simultaneously driving longer-term shifts in hormones.

Related: Pregnancy shrinks parts of the brain, leaving 'permanent etchings' postpartum

The new study revealed that, throughout the day, the subject's overall brain volume decreased, as did the thickness of the cortex, the brain's outer layer. The volume of gray matter, which contains the cell bodies of neurons and the connections between them, fell by an average of about 0.6%.

Two regions of the cortex, known as the occipital and parietal cortices, shrank the most. Changes were also seen in deeper brain structures, including the cerebellum, brainstem and parts of the hippocampus. These parts of the brain are respectively involved in coordinating movement; relaying information between the brain and body; and storing memories.