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."

    研究人員日前指出，中學生每天需要九個小時的睡眠，在開學之前，家長們要幫助孩子逐漸回到上學時的作息時間上來。

    得克薩斯大學公共健康學院的羅伯特•羅伯茲教授于上周五稱，上早課對于中學生來說尤其難過，因為“他們正處于青春期，這一階段他們的生物鐘會發(fā)生變化，喜歡晚睡晚起。”

    他說：“青少年每晚需要保證九個小時的睡眠，但有四分之一的人每晚只睡六個小時或者更少。多數(shù)學校上課時間都很早，而這對于中學生來說是一種最不理想的組合（晚睡早起）。”

    羅伯茲教授的同事、邁克爾•斯摩倫斯克教授說，一項針對馬薩諸塞四所公立中學3100多名學生開展的睡眠研究得出的結(jié)果很有啟發(fā)。

    斯摩倫斯克教授在《生物鐘•健康指南》一書中寫道：“與成績?yōu)镃、D、F的落后生相比，常得A、B的優(yōu)等生在平時和周末晚上的就寢時間都更早。優(yōu)等生平時每晚的睡眠時間比落后生多約25分鐘。”

    羅伯茲建議，“如果學校都改成中午上課，晚上六七點放學，那學生們就好過多了。他們一定會覺得很幸福。當然，老師們就不一定了。”