Over half of U.S. teens kids 15 years and older are sleep deprived and need to sleep at least two hours more each night to meet guidelines for adequate rest.

The recommendation comes from researchers that analyzed the University of Michigan's annual "Monitoring the Future" national surveys of youth behavior. The survey's finding increased concerns about the impact of sleep deprivation on the health and academic performance of American teens.

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The 11-year study beginning in 1991 involved nearly 300,000 teens. It asked teens if they regularly got at least seven hours of sleep nightly. Experts generally recommend nine or 10 hours of sleep for teens.

The survey results published in the magazine, Pediatrics, showed over half of the 15- to 19-year-olds surveyed in 2012 not getting even getting seven hours of sleep each night.

Teens of all ages reported getting less sleep during the two decades covered by the study. The biggest drop was among 15 year-olds with 43 percent reporting at least seven hours of sleep in 2012 compared to more than half in 1991. In most surveys, girls and non-whites were the least likely to report seven hours of sleep.

Reasons for the drop in sleep time aren't clear factors that might have contributed include more use of social media, smartphones and other electronics, and rising rates of obesity that has been linked to sleep deprivation.

Other research suggest that early school start times play a role in this lack of sleep and advocates are pushing for later times for teens. Some 40 percent of U.S. high schools start classes before 8:00 a.m. Experts have described these early morning hours as teens' "optimal sleep period".

Teens that don't get enough sleep are at risk for depression, memory and learning difficulties poor grades and mood problems

The National Sleep Foundation this February updated its sleep recommendations for teenager., It suggests teens ages 14 to 17 need 8 to 10 hours of sleep per night, up from the previous recommendation of 8-1/2 to 9-1/2 hours.