CHICAGO—The American Medical Association on Tuesday, June 14, urged starting school later in the morning for teenagers so they can get enough sleep.

In a new policy adopted on Tuesday, the AMA said middle and high schools should start at 8:30 a.m. at the earliest because research has shown that puberty is accompanied by a biological shift in circadian rhythm that contributes to later bedtimes and wake-up times in adolescents.

The influential doctors group said nearly 10 percent of U.S. high schools today begin at or before 7:30 a.m. as districts try to make time for additional classes, sports and extracurricular activities.

Inadequate sleep has been linked to a host of mental and physical problems, from poor memory performance and mood disorders to impaired immune function and unhealthy body mass index, the AMA said.

"Sleep deprivation is a growing public health issue affecting our nation's adolescents, putting them at risk for mental, physical and emotional distress and disorders," said Dr. William Kobler, an AMA board member.

"Scientific evidence strongly suggests that allowing adolescents more time for sleep at the appropriate hours results in improvements in health, academic performance, behavior, and general well-being," he added.

Only 32 percent of American teens in a recent study reported getting at least eight hours of sleep on an average school night. The American Academy of Pediatrics recommends teenagers between 14 and 17 years old should get 8.5 to 9.5 hours of sleep per night.

The AMA's new policy, adopted at the group's annual meeting in Chicago, also encourages physicians to educate parents, school administrators and teachers about the importance of sleep for adolescent mental and physical health.