Now that New Hampshire is voting, the candidates must make hay in South Carolina and Nevada while also preparing for Super Tuesday, just three weeks from today, when 14 states choose convention delegates. Taking a break only damages your chances. It’s the world’s longest, highest-stakes ironman triathlon. That’s why, long ago, the political operatives who worked on the campaigns of John, Robert, and Edward Kennedy would check themselves into a hospital on the day after election day.

Our topic today is stress because it’s a centrally important issue for all leaders and it is, like sleep, an issue that big-deal leaders are supposed to laugh at. Sleep? Who needs it? Donald Trump claimed in a debate that he got along fine on three hours a night. Stress? It goes with the territory. Don’t whine about it. But in reality stress hurts you and can kill you, and, like sleep deprivation, it’s an issue that all leaders need to confront.

You know that stress can give you a headache, muscle aches, and indigestion, but it also does much worse. Chronic stress, which is the life of many business and political leaders, can cause anxiety and clinical depression and can be a factor in drug abuse and social withdrawal. These are not trivial possibilities; you may recall a rash of suicides among bankers in the last recession and again between August 2013 and February 2014, many of which were linked strongly to extreme stress. An article in today’s WSJ observes that stress is often an overlooked factor in high cholesterol.

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We have no specific reason to believe that stress played a direct role in the heart attack of United Continental CEO Oscar Munoz in October, a few weeks after he was suddenly given the job, or in the hospitalization of Valeant Pharmaceuticals (VRX) CEO J. Michael Pearson with pneumonia in December. But both men were in extremely stressful situations, and even if stress didn’t cause their health problems, it can slow recovery from any illness. Munoz has since had a heart transplant, and United Continental (UAL) says he’ll be back by the end of March. Valeant has named an interim CEO and has not said when Pearson will return to work.

The larger point is that leadership is a physical job, though we rarely think of it that way. When Hedley Donovan stepped down as editor-in-chief of Time Inc. (Fortune’s parent) in 1979, he was asked to name the most important trait for the role. He said nothing about intellect or editorial judgment; “stamina” was his answer. Leaders need to accept that stress is a genuine danger and must be addressed like any other. More immediately, the evidence suggests there’s good reason to be concerned about the health of the presidential candidates. Let’s hope that all of them, even the ones you hate, make it.