A 30-second cold blast at the end of your shower cuts sick days by 29 percent. A 2016 trial of 3,018 adults in the Netherlands is the largest cold-shower study ever done. Geert Buijze at the Academic Medical Centre in Amsterdam split participants into four groups: 30, 60, or 90 seconds of cold water at the end of a shower, or no cold water. They did this for 30 days straight. All three cold groups had 29 percent fewer sick days. The length did not matter. Thirty seconds worked as well as ninety. The mechanism is thought to involve the immune system. Brief cold exposure triggers a spike in white blood cell count and an increase in metabolic rate, both of which are part of the body's response to what it perceives as a physical threat. Over time, repeated cold exposure appears to train the immune system to respond faster and more efficiently to infections. The study also found that 91 percent of participants in the cold-water group said they would continue the practice after the trial ended, even though most of them described the experience as unpleasant. They reported feeling more energetic, and the energy boost was the reason they kept going. Thirty seconds. At the end of the shower you were already taking. The water does not have to be ice cold. It has to be cold enough to make you gasp, and you have to stay in it for a count of thirty.