SLEEP 139 



our surroundings and deep sleep succeeds, to be fol- 

 lowed by a lighter slumber until this in turn gives way 

 to a condition quite similar to drowsiness, a half con- 

 sciousness of our surroundings, an arousing of atten- 

 tion until we are completely conscious or "awake." 

 Sleep, in healthy adults, usually lasts from seven to 

 eight hours, though the time given or required varies 

 greatly with the individual and with age. Newborn 

 infants sleep from eighteen to twenty hours a day, old 

 people five or six. In other words when growth is most 

 active, sleep is most needed. In the old there is no 

 growth and comparatively little repair, while, during 

 the active period of life, though growth has stopped, 

 repair is steady and imperative. 



The depth of sleep varies greatly. Normally it would 

 seem that the first two hours are characterized by the 

 deepest sleep, which gives way to a lighter type which, 

 in its turn, is succeeded by more profound unconscious- 

 ness a short time before waking. Nearly all secretions 

 are diminished during sleep. The tears, which moisten 

 the eyeballs and lids, are nearly absent even in drowsi- 

 ness, the dryness of the mouth and throat on waking 

 are evidence of the lack of salivary secretion, while the 

 high color and comparatively small amount of urine 

 accumulated during the night, show that at least the 

 watery part of the urine is decreased. On the contrary 

 perspiration increases to such an extent that it has been 

 asserted that as much is produced during seven sleeping 

 as in fourteen waking hours. Respiration is slower in 

 sleep and often becomes intermittent, particularly in 

 children and the old, and the costal type of breathing 

 is the rule. The heart beats more slowly, though it is 

 said to increase in rapidity near the waking hour. Su- 

 perficial blood vessels are dilated and congested, while 



