Exercising before something stressful makes your body react less to the stress. In a Berlin study, young men who exercised for 30 minutes released less of the stress hormone cortisol during a tough test 90 minutes later. One group of men did 30 minutes of moderate exercise on a treadmill, and the other did 30 minutes of very light movement, with fit and inactive men assigned at random. An hour and a half later, each man lay in a brain scanner and did mental math under time pressure, with feedback designed to make him feel like he was failing. The study appeared in the journal Psychoneuroendocrinology in 2015. The men who exercised had a smaller cortisol spike during the test. Exercise raises cortisol for a while, and the body seems to respond by turning its stress system down, like a built-in brake. The men whose stress hormones rose the most during exercise had the smallest spike during the test. Brain scans showed more activity in the exercisers' hippocampus, a memory center, and less at the front of the brain, two areas that help control stress hormones. Fitter men also had smaller cortisol spikes in general. Only young men took part, so women and older adults might respond differently. About 30 minutes of moderate exercise, an hour and a half before a presentation, exam, or hard conversation, was enough to shrink the stress response in this group.