All day, as we work and think, we store up more poisons in our cells than the body can get rid of, and the time comes when the cells are so loaded with poisons that we have to stop for a while, and let our blood-stream clean house. The quantity of sleep one needs is a problem like that of cold water; each person has to find his own rule. In general, one needs less and less sleep as one grows older. Infants sleep the greater part of the time; growing children should sleep ten or eleven hours, adults seven or eight, and old people, unless they have let themselves get fat, generally do not want to sleep more than six, and part of this in short naps. When you sleep, your bodily energies relax, and you make less heat, therefore you need extra clothing; but this clothing should never cover the mouth and nose, nor should it be so heavy as to make breathing a burden. If you are in good condition, it will do you no harm to be chilly when you sleep, except that you do not sleep so soundly. Sleeping too much is just as harmful as sleeping too little. Nature will tell you that. The important thing, as in all other problems of health, is to have something interesting to think about, some exciting work to do in the world, and then you will sleep as little as you have too. CHAPTER XXIV WORK AND PLAY (Deals with the question of exercise, both for the idle and the overworked.) In discussing the important question of exercise, there is one fundamental fact to begin with: that our present civilization divides men sharply into two classes, those who do not get enough exercise, and those who get too much. Obviously it would be folly to make the same recommendations to the two classes. I begin with those who get too much exercise. They include a great number, probably the majority of those who do the manual work of the world. They include the farmers and the farm-hands, who work from dawn to sunset, and sometimes by lantern light. They include also the farmers' wives, the kitchen slaves of whom the old couplet tells: "Man's work ends from sun to sun, But woman's work is never done." I am aware that men have worked that way for countless ages, and yet the race is still surviving; but I am aware also that men wither up with rheumatism, and contract chronic diseases of the kidneys and the blood vessels, consequent upon the creation of greater quantities of fatigue poisons than the body can regularly eliminate.