Enough sleep is important to health. The amount of sleep（21）depends on the age of the per-son and the conditions in which sleep（22）. The young may need more sleep than the old, but generally eight hours are enough for the health of grown-ups. Some can（23）with less than this amount but（24）may need more. Every person knows his own need. It is then a matter of how to（25）it. Sleep should always be enough to make one relaxed(松弛)and ready for（26）work.
Fresh air is（ 27）to sound sleep(酣睡). It is not without reason for some people to（28）that it is practical to sleep in the open air.（29）a person can keep himself warm, out-of-door sleeping probably gives the body（30）complete relaxation.
Ability to sleep is largely a habit. The conditions referred to only lead to sleep. Out-of-door（ 31）,a good habit of regular drinking and the avoidance of late eating and（32）are all helpful to sound sleep. Such factors are largely within the control of any person. A bath at（33）, neither hot nor cold but of（34）temperature, may be helpful to sleep. Sleeping pills should never be taken except when suggested by（35）.
第（21）题选