A new study shows that the early start of classes in most public schools results in teenagers not getting enough sleep. As a result, the researchers claim that students are vulnerable to general moodiness and performing poorly in school. The results of the study were published in the journal 'Pediatrics.'

The study examined the sleeping habits of 60 teens who kept a sleep diary. They kept the diary one month over summer vacation when school was not in session and several months later during the school year.

The results indicate that students between the ages of 12 and 15 lost an average of two hours of sleep per night on school nights which affects their ability to work and concentrate in school.

According to the study, adolescents are undergoing changes to their biological clocks at around the same time they enter high school. Those changes, called circadian rhythms, dictate when a person becomes sleepy and when they are fully awake. Teenagers bodies tell them to go to sleep later and wake up later.

However, 85 percent of American high schools begin classes before 8:15 AM with more than one-third starting at 7:30 AM or earlier.

In addition to problems concentrating in school, chronic lack of sleep can make teens more likely to begin smoking and more likely to be involved in a motor vehicle accident.

The study concluded that '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.'

The Northwestern University study believes that the hours teens sleep on weekends are more in tune with their natural body chemistry. And you thought your teen was just lazy...