xvn CARE AND TRAINING OF THE NERVOUS SYSTEM 247 



slower, the temperature falls a little, and the activity cf 

 all the cells is at a minimum. But certain very impor- 

 tant changes occur that make sleep absolutely necessary 

 to life and health. Let us see what they are. (i) The 

 heart, lungs, and certain other organs are at work all 

 night, slowly removing from the cells and the blood the 

 waste products accumulated during the day. (2) The 

 brain cells are repaired so as to render them more 

 responsive to the sensory impressions or stimuli that are 

 carried to them. (3) The cells are supplied with an 

 abundance of food material for future use. (4) Not 

 only the brain, but all the organs and tissues of the body 

 are renovated, repaired, and stored with energy-yielding 

 materials. Of course these changes can occur only 

 when the blood contains an abundance of nutritious 

 food stuffs and an ample supply of oxygen. 



The amount of sleep required varies with age, occupa- 

 tion, and conditions of health, and may depend some- 

 what on personal habits. The adult needs about eight 

 hours in every twenty-four for sleep, while the child 

 under five should have at least twelve hours. The brain 

 worker needs a little more than the laboring man. 

 Sleep should be taken at the same time every day. If 

 for any reason the person does not get the required 

 amount of sleep, it shows itself in both mental and 

 muscular work. For children and youths loss of sleep 

 is a very serious matter, because, as long as they are 

 growing, all the cells of the body are laying up a supply 

 of energy for all future years. 



Some persons find it difficult to go to sleep, and resort 



