Enough sleep is important to health. The amount of sleep _____（21）depends on the age of the person and the conditions in which sleep _____（22）. The young may need more sleep than the old, but _____（23）eight hours is enough for the health of grown-ups. Some can do 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 be always enough to make one relaxed（松弛）and ready for _____work.Fresh air is _____（27） to sound sleep（酣睡）. It is not _____（28）reason for some people to insist that it is practical to sleep in the open air. _____ a person can keep himself warm, out-of-door sleeping probably gives the body _____ 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 _____（33）of any person. A bath at _____（34）, neither hot nor cold but of body temperature, may be helpful to sleep. Sleeping pills should never be taken except when suggested by _____（35）.22_____.