Teenagers need nine hours of sleep a night and parents can help by getting them back on a school sleep schedule before classes begin, researchers suggest.

    Early morning classes can be particularly hard on teenagers because "their circadian rhythms change at puberty and they want to go to sleep later and wake up later," Robert Roberts, a professor at the University of Texas School of Public Health, said on Friday.

    "Adolescents need nine hours of sleep a night and about a fourth of them get six hours or less," he said. "Most classes begin very early and for adolescents that's the worst combination."

    A sleep study involving more than 3,100 students at four Massachusetts public high schools produced some eye-opening results, according to Roberts' colleague, Professor Michael Smolensk.

    "The top students, the ones earning mainly A's and B's, went to bed earlier on both weeknights and weekends than those who received C's, D's and F's. The high achievers slept about 25 minutes longer on school nights than did the low achievers," Smolensk wrote in his book, "The Body Clock, Guide to Better Health."

    Roberts suggested that "students would be better off if high school classes started at noon and ended six or seven in the evening. They'd be much happier. Of course, their teachers might not be."

    研究人员日前指出，中学生每天需要九个小时的睡眠，在开学之前，家长们要帮助孩子逐渐回到上学时的作息时间上来。

    得克萨斯大学公共健康学院的罗伯特•罗伯兹教授于上周五称，上早课对于中学生来说尤其难过，因为“他们正处于青春期，这一阶段他们的生物钟会发生变化，喜欢晚睡晚起。”

    他说：“青少年每晚需要保证九个小时的睡眠，但有四分之一的人每晚只睡六个小时或者更少。多数学校上课时间都很早，而这对于中学生来说是一种最不理想的组合（晚睡早起）。”

    罗伯兹教授的同事、迈克尔•斯摩伦斯克教授说，一项针对马萨诸塞四所公立中学3100多名学生开展的睡眠研究得出的结果很有启发。

    斯摩伦斯克教授在《生物钟•健康指南》一书中写道：“与成绩为C、D、F的落后生相比，常得A、B的优等生在平时和周末晚上的就寝时间都更早。优等生平时每晚的睡眠时间比落后生多约25分钟。”

    罗伯兹建议，“如果学校都改成中午上课，晚上六七点放学，那学生们就好过多了。他们一定会觉得很幸福。当然，老师们就不一定了。”