| A recent study conducted by researchers at the University of Michigan ' s Center for Human Growth and Development shows that children aged 9 t0 12 who sleep less than nine hours a night can have real problems with gaining weight. Dr.Julie Lumeng, lead author of the report said, " Many children aren't getting enough sleep , which may not only make them annoyed or prevent them from being active and ready to learn at school,but also lead to a higher risk of being overweight. " Reuters reports. She also said that children who sleep more than 10 hours a night are more willing to play and consume their energy , rather than staying in bed or watching TV as their tired fellows do. It is also said that hormonal(荷尔蒙) changes brought on by a lack of sleep can lead to an increased appetite. The study focused on 785 children aged 9 to 12 from 10 different states and it confirmed that 18% were overweight by the time they reached the sixth grade. Children who do not sleep nine or more hours a night when they are in the third grade are more likely to gain weight by the time they are in the sixth grade. The conclusion of the study is very simple : "Our findings also provide additional support for policies that approve of later school start times. The very early school start times for US children have raised concerns in the medical community because of their apparent negative effects on sleep duration(廷续性) and , consequently, children's general academic and behavioral functioning , " Reuters reports. According to National Sleep Foundation, children in first grade should sleep from 10 to 12 hours a night. Dr. Julie Lumeng' s report was published in the journal of the American Academy of Pediatrics (小 儿辩 ) in November. | 
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