_Accumulated fatigue or physical debt._ If you go up a long flight of steps at a moderate rate, you can get to the top without being tired; if you go up at a rapid rate, as most of us do, you are tired at the top. Physically you put out the same amount of energy, I suppose. I do not see that there can be any considerable difference in the energy consumed by the performance of that act whether we do it slowly or quickly. The difference is that in the first case we _rest between each two steps_ as we rest between each two days at night. When our activities are so balanced as not to run in debt, we rest between each two steps. You and I can walk at our individual peculiar gait on the level for a long time without any accumulation of fatigue, often with refreshment. But push us and we are soon exhausted. Suppose that our normal walking rate is three and a half miles an hour; push us to four, and it may not be a quarter of a mile before we are done up, because we have not been able to avoid accumulated fatigue by resting between each two steps. It has been said that in rowing the crew that wins is the crew that rests between each two strokes. The person who does not get tired is the person who rests between each two days. He does not accumulate fatigue. It is the accumulation that finally breaks you, makes you bankrupt. It is the little unnoticed bit added day by day, week by week, month by month, that makes the break. Fatigue we should think of as running in debt. One of the figures of speech that has served me best in teaching patients how to live is that figure of income and outgo. I have often said to people, "Physically you are spending more than you earn, not to-day merely, but right along. You must earn more than you spend. You must get a plus balance in the bank. Then you can run along with fatigue or illness." That figure of speech helps us also to express another fact about fatigue, which is important to recognize in ourselves and in our patients, because otherwise we get thrown off the track: _delayed fatigue_. The first day that your income begins to be less than your expenditures, nothing necessarily happens. The bank does not proclaim that there is no deposit there. It is some days later, usually, that you begin to reap your troubles. It is the same in physical fatigue. Patients say to us, "I slept ten hours last night. I spent a virtuous Sunday. Why should I be tired to-day?" We should answer, "Because of something you did last Tuesday or thereabouts." We all are familiar with this in relation to sleep. It is not the day after a bad night, but several days later that its effects depress us.