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.

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."