Sleepy teens haven't got circadian rhythm.

High schools that begin classes as early as 7:30 a.m. deprive teenagers of sleep, and attempts to reset an adolescent's biological clock fail to solve the problem, a study in the June Pediatrics finds.

Sixty high school students in Evanston, Ill., recorded their sleep times in diaries in August, September, and November of 1997 and in February 1998. The diaries showed that students stayed up late in August but still managed to sleep 8 to 9 hours. When school started, they continued staying up late but had to awaken early for school, says Margarita L. Dubocovich, a neuropharmacologist at Northwestern University in Evanston. The teens' average sleep fell to 6 to 7 hours on weekdays.

In an attempt to reset the students' daily biological clocks, or circadian rhythms, so that they would be more alert in daytime and go to bed earlier, the researchers exposed some students in their classrooms to especially bright light between 8 a.m. and 10 a.m. Other students were exposed to muted red lighting. But the bright light neither changed students' sleep patterns nor improved their scores on tests of mood, vigor, and cognitive function.

Hormones, television watching, Web surfing, and other factors might explain why adolescents delay sleep in the evening, Dubocovich says, adding that "it's not well understood."

Dubocovich advocates napping in the afternoons and says high school officials should consider starting classes later and scheduling important tests only for afternoons. Only 16 percent of high schools in the United States start later than 8:15 a.m., says Richard P. Millman of Brown University in Providence, R.I., also reporting in Pediatrics.--N.S.