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 26 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. 29 a person can keep himself warm, out of-door sleeping probably gives the body 30 complete relaxtion.
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 .
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通过上句话中的grown-ups，可推断出本题的答案应是children。
