Ben Franklin was right, at least on the healthy part. "Early to bed and early to rise" appears to have helped a cross-section of early-bird Australian youths keep slimmer and more physically1 active than their night-owl peers2, even though both groups got the same amount of sleep. A study in the Oct. 1 issue of the journal Sleep recorded the bedtimes and wake times of 2,200 Australian participants, ages 9 to 16, and compared their weights and uses of free time over four days. Children who went to bed late and got up late were 1.5 times more likely to become obese3 than those who went to bed early and got up early. Furthermore, late-nighters were almost twice as likely to be physically inactive（不活跃的） and 2.9 times more likely to sit in front of the TV and computer or play video games for more hours than guidelines4 recommend.
"The children who went to bed late and woke up late, and the children who went to bed early and woke up early got virtually the same amount of sleep in total," said co-author Carol Maher, PhD, a postdoctoral fellow with the University of South Australia. "Scientists have realized in recent years that children who get less sleep tend to do worse on a variety of health outcomes5, including the risk of being overweight and obese. Our study suggests that the timing6 of sleep is even more important."
Maher said mornings are more conducive7 to physical activity for young people than nights, which offer prime-time TV programming and social networking opportunities. This relationship between time of day and available activities might explain why more sedentary（久坐的） and screen-based behaviors were observed with later bedtimes, she said. At a time when research is showing that teenagers have a natural tendency to stay up late（熬夜） and wake late, the results of this study could stand as a warning.
"It is widely accepted that the sleep patterns of adolescents are fundamentally different from children and adults, and that it is normal for adolescents to stay up very late and sleep in late in the morning," Maher said. "Our findings show that this sleeping pattern is associated with unfavorable activity patterns and health outcomes, and that the adolescents who don't follow this sleep pattern do better."
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