Dr. Geert A. Buijze, of the Netherlands, has conducted a high level study involving 3,000 volunteers, showing high level evidence that cold showers of 30-90 seconds can improve health, reducing absences from work by 29%. Cold showers trigger the neuroendocrine effect of fight or flight, which increases the cortisol before it shifts into the relaxation response, activating the good, or brown, fat in the body. When the brown fat is activated, it keeps the body warm by burning calories, which may help to control blood sugar, thereby reducing the risk of obesity and diabetes. Two-thirds of the participants continued the cold showers after the study was completed.
~Harvard Business Review, March-April 2018