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.
Delayed fatigue, then, is an important thing to notice in ourselves and to bring home to the people that we are trying to help. I suppose one could say that _a great part of our business in social work is to call people's attention to things_; if they have recognized them before, they will perhaps get a lesson out of what we say. Such matters are _referred fatigue_, _delayed fatigue_, _accumulated fatigue_,--familiar enough, only the person does not act on them because he does not notice them.
The fatigue-rest rhythm, the alternation of fatigue and rest, I have already phrased by the metaphor of earning and spending. You can phrase it also by a metaphor very close to the physical facts as we know them, the metaphor of building up and tearing down. During the daytime, from the point of view of physiology and the workings of the body, we burn up tissue. In us oxidation processes are going on which are really burning, as really as if we saw the flame. Tissue is being destroyed, broken down, going off in the form of heat, energy, and life. That is good in case it is followed, as it should be, by a period of rest in which we build up. Presumably, if we could see with adequate powers of the microscope or powers of observation of some sort, what goes on during rest, we should see a perfect fever of rebuilding all that we have torn down during the day. People often say, "Shall I take exercise?" Yes, but remember that half of the process of taking exercise is getting rested afterwards. It will do you good provided you rest after it, provided what has been torn down in exercise is replaced by sufficient tissue or fresh power in rest.