This metabolic rhythm impresses itself on all group activity, and no one can be a successful "social engineer" who does not take account of it. The public speaker allows times in his address when his hearers may relax their attention or change the kind of mental process which he requires of them, and herein is the real reason for the jokes and anecdotes with which a long address is interspersed. A teacher does the same thing in a recitation by having a variety of work done. To the same end, the school program combines periods for study, manual training, recitation, gymnastics, and play.

Some of the longer periods of the metabolic rhythm are synchronized with those of nature. The earth's daily rotation makes a cycle which has become inherent in the constitution of every living thing, of every person, and of every form of social life. The school assembles in the morning, has "morning exercises," and goes through those forms of work which demand the highest degree of mental efficiency; then there is an interval for lunch, and then the afternoon and evening have their appropriate exercises. The daily round repeats itself with more or less of regularity. The weekly cycle does not appear to correspond to anything in organic nature, but it probably has a metabolic basis else it would not be so prevalent. The lunar month is a cycle in nature from which the month of our calendar is derived; it is therefore a cycle with which many social arrangements are timed, such as the payment of salaries, and the making of reports. The cycle of seasons resulting from the annual revolution of the earth around the sun forces human society everywhere through a corresponding cycle of important changes which vary according to the climate of the particular locality.

The principle involved in all the forms of relaxation ... is relief from tension or release from some form of restraint. Although this tension and restraint on the part of the individual are necessary conditions of all social evolution, they have been greatly intensified by the manner of life which characterizes the nineteenth and twentieth centuries. . . .

When this everlasting urge of progress is excessive, as it has been in recent times, we may say that there is in a way a constant subconscious rebellion against it and a constant disposition to escape from it, and the method of escape is always the temporary reversion to simpler and more primitive forms of behavior, - a return to nature, so to speak. Sudden momentary and unexpected release from this tension, with instinctive reinstatement of primitive forms of expression, is laughter. Daily or periodic systematic return to primitive forms of activity is sport or play. War is a violent social reversion to elemental and natural intertribal relations. Profanity is a resort to primitive forms of vocal expression to relieve a situation which threatens one's well-being. Alcohol is an artificial means of relieving mental tension by the narcotizing of the higher brain centers. - Patrick, The Psychology of Relaxation, pp. 18-20.

. . . The course of annual rainfall in the great cereal-producing area of the United States has been shown to move in cycles: there is a ground-swell of thirty-three years in length upon which cycles of eight years in duration are superposed.

. . . The rhythm in the activity of economic life, the alternation of buoyant, purposeful expansion with aimless depression, is caused by the rhythm in the yield per acre of the crops; while the rhythm in the production of the crops is, in turn, caused by the rhythm of changing weather which is represented by the cyclical changes in the amount of rainfall. ... - Moore, Economic Cycles, pp. 36, 135.