Key Findings: New research from reveals that the gut microbiome regulates the body’s diurnal (day-night) rhythms in stress hormones. Depletion of gut microbiota results in disruptions in the brain’s core circadian system, and is associated with altered stress hormone rhythms. The research highlights that gut microbes also regulates key stress-responding brain regions throughout the day. Using microbe transplantation, the team confirmed that oscillations of gut microbes across the day are critical for regulating stress hormone secretion.