There are not a lot of experiments on cold showers because think about it's very hard to control. When you get into cold water, that's the shock. The first 30 seconds for most people who are untrained, your forebrain is basically suppressed in its activity and other areas are ramped up. In that long arc of dopamine, it leads to a near-doubling or more of dopamine and epinephrine. That's what was shown in the study.