How tweens use their electronic devices before bedtime can have a major impact on how well — and how long — they sleep, according to a new study.

The study involved nearly 9,400 11- and 12-year-olds who are part of the Adolescent Brain Cognitive Development Study, a large, long-term study of brain development and child health in the United States.

Researchers asked the tweens and their parents detailed questions about how they used screens around bedtime and how well they slept; the researchers then followed up a year later. A majority of the tweens in the study — 72% — had their own cellphone.

One surprising finding: Silencing phone notifications before bedtime was associated with less sleep than turning off the phone completely, says Dr. Jason Nagata, an associate professor of pediatrics at the University of California, San Francisco, who led the study.

"So if your phone is not completely off, teens may still be woken up overnight by vibrations or lights," Nagata says. "Or if they have any difficulty falling asleep, it might be easier for them to just check their phones if they're bored or have any sleep disruption."

One out of five tweens in the study reported checking their phones if they woke up overnight, which Nagata says can lead them to become more awake or aroused, making it harder to fall back asleep.

He says a better strategy is to keep smartphones out of the bedroom altogether overnight: The study found that adolescents who did so got more and better-quality sleep, a finding that persisted over the year.

Overall, using electronic devices for any purpose while in bed — whether to text, stream videos or TV shows or play games — was associated with less sleep.

"I think the key take-homes for parents or preteens are to just avoid social media or any electronic devices, especially an hour before bedtime," Nagata says. "If it's possible to keep the devices outside of the bedroom, I think that's the most effective."

Nagata and his colleagues say their research points to several best practices for parents.

1. No screens for an hour before bedtime.
2. Turn phones all the way off at night.
3. Resist the urge to check the phone if you wake up in the middle of the night.
4. Keep phones outside the bedroom at night.

And that advice goes for grownups too.
"After doing these studies, I have changed my habits," Nagata says. "I used to have my phone off, but in the room. But now I try to put it outside the bedroom, and it has actually been more effective for me as well."

The research appears in the Journal of Adolescent Health.

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