New research shows teens not getting nearly enough sleep

New research shows that many teenagers are not getting enough sleep.

Some are getting less than five hours a night, which can have serious effects on their health.

From school to activities, teenagers are tired.

A new study of more than 120,000 high school students finds that nearly 77% are missing the mark on the recommended amount of sleep on school nights, up significantly from 69% when the study began in 2007.

But the most dramatic shift is in the kids getting the least amount of sleep. Nearly one in four teens now reports sleeping five hours or less per night.

"There is biology there that says it's harder when kids enter those teen years to get to bed on time," said Tara Narula, ABC News Chief Medical Correspondent.

The trend was seen across grades, sexes, and ethnic groups, though increases were larger among non-Hispanic Black teens.

Researchers found that teens who report depression symptoms, bullying, or suicidal thoughts were more likely to sleep less, but sleep loss also rose among teens without those factors.

Researchers say the trend likely reflects broader pressures in teens' daily lives, not just screen use.

Though a separate review of more than 4,500 children and young adults found that more overall screen time was linked to later bedtimes, but not necessarily worse sleep quality or shorter duration.

However, using screens once already in bed was associated with worse sleep that night, supporting what researchers call the "sleep displacement hypothesis" - meaning screen time can crowd out time meant for sleep.

"So, parents can make sure screens are out of rooms, try to get the kids to turn the screens off before bedtime, make sure they're exercising, getting into sunlight during the day, and try to keep a consistent bedtime week and weekend," said Narula.

The CDC says a recent study found that about 15% of teens report feeling tired most days of the week. They recommend teens get eight to ten hours of sleep per night.

The Journal of the American Medical Association points to school start times as a major factor in insufficient sleep, saying studies suggest a later school start time, ideally 8:30 a.m. or later, leads to longer sleep times for teenagers.