According to the National Sleep Foundation,

　　41 should sleep twelve to eighteen hours

　　42

　　of every twenty-four. With a gradual

　　43

　　to twelve to fourteen hours for toddlers(初学走路的孩子) one to three; eleven to thirteen hours for

　　4

　　three to five; and ten to eleven hours for schoolchildren

　　45

　　five to ten.

　　But things get really

　　46

　　at adolescence. Not only do teenagers need more sleep than

　　47 __---eight and a half to nine and a quarter hours a night, according to the sleep foundation - but the times

　　48

　　which they get sleepy and are able to awaken

　　49

　　and feel rested shift in a

　　50

　　that does not match to the

　　51

　　times at most schools.

　　Sleep studies have shown that the typical teenager does not fall asleep readily before 11 p. m. or later.

　　52

　　many have to get up by 6 a. m. or

　　53

　　to get to school for a class

　　54

　　starts at 7:30 or 8 a.m. More than a few

　　55

　　off during that class, and often the

　　56

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