322 PHYSIOLOGY AND HEALTH. 



market-men in the neighborhood of cities, and workmen in 

 tide-mills, spend a part of the night in their business, and 

 make up their loss of sleep in the day ; but in a few years they 

 are very glad to discontinue this course, and confine themselves 

 to daylight labor. Mr. G. owned and worked a tide-mill, which 

 could run only for a few hours succeeding a full tide, which 

 came as often in the night as in the day. In course of a few 

 years, this frequent night work wore so much upon him as to 

 compel him to exchange his tide-water power for steam 

 power, which he could use to suit his own convenience. 

 Having given up night work, and limited his labor to the 

 day, from a feeble he has become a robust man, and is able 

 to accomplish more in the new than in the old system. 



740. It is a law of nature that all animals shall suspend 

 their actions, and sleep. The alternations of day and night 

 harmonize with this want of the living animal body, and 

 afford seasons of activity and of rest. Man needs to follow 

 this natural indication, and alternate his sleep and wakeful- 

 ness daily. Sleep is nature's restorer of exhausted power, 

 and, though we retire wearied, we awake refreshed and 

 strong; the expended energies are recovered, the strength 

 brought back, and we are again ready for action. In the 

 state of sleep, all motion of the voluntary muscles is stayed, 

 and the brain suspends its active functions ; but the invol- 

 untary functions go on as when awake; the chest moves, 

 the lungs breathe, and the blood is purified, the heart beats, 

 the blood circulates, and the system is nourished. 



741. The quantity of sleep that is necessary is varied by 

 ,so many circumstances, that no rule can be established for 

 all. The time of life and the peculiarities of constitution 

 make a difference. The sluggish and the lymphatic need 

 more sleep than the active and the nervous. Some sleep very 

 much, and are not refreshed, nor ready for action, if they 

 are deprived of their usual quantity of rest. Others take 

 very little sleep, and cannot obtain more. Without giving 

 any precise rule, it is sufficient to say that men and women 

 who have arrived at .adult years, and developed their full 



