The early start to classes causes teenagers to lose much needed sleep that puts them at risk of becoming moody and performing poorly in school, researchers said.

A survey of students aged 12 to 15 years old found they lost an average of two hours of sleep on the nights before a school day, affecting their work and alertness in class.

More than one-third of high schools rang the bell to begin classes at 7:30 a.m. or earlier, a separate 2003 survey found, and 85 percent of high schools begin the day before 8:15 a.m., the report said. Students living long distances from school face even earlier wake-up calls to catch rides.

Sleepiness is associated with moodiness and poor school performance, and a chronic shortage of sleep correlates with a greater propensity to take up smoking or to be involved in car crashes, the researchers said.

At the time adolescents enter high school their bodies are undergoing changes to their biological clocks -- called circadian rhythms -- that govern when they become sleepy and fully awake. The changes tend to make them go to bed later and wake up later, said the report published in the journal Pediatrics.

In the study, 60 teens kept sleep diaries during a month in summer when schools were closed and again during a few months of the school year.

While they lost sleep on school nights, they slept 30 minutes longer than usual on weekend nights during the school year and on summer nights, indicating they were not just using weekends to catch up on sleep during the school year.

"Weekend sleep and weekend wake times may represent a return to an intrinsic circadian rhythm that is lost during the week because of the imposed school schedules," said the study by researchers at Northwestern University and Evanston Township High School in Illinois.

The findings point to sleepy students in the mornings who tend to become more alert in the afternoons. Teachers attested to this in an accompanying report published in the journal and written by pediatric sleep experts.