Enough sleep is important to health. The amount of sleep 【C1】______ depends on the age of the person and the conditions in which sleep 【C2】______ . The young may need more sleep than the old, but 【C3】______ eight hours are enough for the 【C4】______ of grown-ups. Some can 【C5】______ with less than this amount but 【C6】______ may need more. Every person knows his own 【C7】______ It is then a matter of how to 【C8】______ it. Sleep should always be 【C9】______ to make one relaxed and ready for 【C10】______ work. Fresh air is 【C11】______ to sound sleep. It is not 【C12】______ reason for some people to 【C13】______ that it is practical to sleep in the open air. 【C14】______ a person can keep himself warm, 【C15】______ sleeping probably gives the body 【C16】______ complete relaxation. Ability to sleep is largely a 【C17】______ . Out-of-door 【C18】______ , a good habit of regular 【C19】______ , the avoidance(避免)of late eating, and 【C20】______ are all helpful to sound sleep. 【C1】______