Work carried beyond the point of normal fatigue requires a proportionately longer time for recovery. For example, if the point of fatigue has been reached by a certain finger muscle after 15 contractions, and if half an hour is required to rest it completely, one might suppose that one hour would rest it after 30 contractions. This is not so, however; after 30 contractions 2 hours are required, or 4 times as much rest for twice the amount of work, if continued beyond the point of fatigue. Laboratory experiments and experience alike show that this principle holds true in other forms of fatigue. Thus the output of factories has been shown in many instances to be greater, other things being equal, when operatives work 8 hours a day than when they work longer. Excessive hours in any kind of work are the poorest economy. Fatigue is increased in direct proportion not only to muscular exertion but also to the amount of speed, complexity, responsibility, monotony, noise, and confusion involved in an occupation. Ability to bear fatigue differs greatly with different people, as ability varies to bear other kinds of strain. Rest at night and on Sunday, and the annual vacation should be enough to keep a person in good condition. If not, there is probably something wrong with the worker's health, the nature of his work, or his adaptation to his particular kind of work. This statement is not only true of persons regularly employed, but of those living at home, including children in school, women in "society," and especially mothers of families. SLEEP.--A sufficient amount of sleep is essential to health, but individual requirements vary widely. Each person should know and regard his own need, and children and young people should be obliged to go to bed early. Ability to sleep is largely habit; good habits should be formed and continued. Sleep-producing drugs should never be taken, except by a doctor's prescription. RECREATION.--Owing to the speed, complexity, and worry of modern life among all classes, and to the monotony of work in industry, recreation has become a matter of vital importance for everyone. Some muscular activity, preferably in the open air, is needed by every healthy person. Recreation should be as unlike the regular occupation as possible: going to the theatre, for example, is not the best exercise for sedentary workers employed all day in artificially lighted offices. The element of pleasure is essential. Hoisting dumb-bells purely from conscientious motives is seldom beneficial, and is generally soon abandoned.