CHICAGO (AP) — A 20-year study says U.S. teens are getting sleepier. The result show many lack even seven hours of shut-eye each night and the problem has worsened over two decades.

The research involved nearly 300,000 teens asked in surveys since 1991 if they regularly got at least seven hours of sleep nightly. They were also asked if they got enough sleep.

Experts generally recommend nine or 10 hours of sleep for teens; over half of the 15- to 19-year-olds surveyed in 2012 said they didn’t even get seven hours each night.

The results heighten concerns about the impact of sleep deprivation on teens’ health and academics.

Results were published in Monday’s Pediatrics.