Teens should limit smartphone usage to two hours a day to get proper sleep: Study

Adolescents today are increasingly depriving themselves of sleep, and instead spending more time on their smartphones or other devices, say scientists who found that teens sleep fewer hours per night than older generations.

Most sleep experts agree that adolescents need nine hours of sleep each night to be engaged and productive students; less than seven hours is considered to be insufficient sleep.

Researchers from San Diego State University and Iowa State University in the US examined data of more than 360,000 US school students who took part in a survey.

The study, published in the journal Sleep Medicine, found that about 40 per cent of adolescents in 2015 slept less than seven hours a night, which is 58 per cent more than in 1991 and 17 per cent more than in 2009.

Delving further into the data, researchers learned that the more time young people reported spending online, the less sleep they got.

Teens who spent five hours a day online were 50 per cent more likely to not sleep enough than their peers who only spent an hour online each day.

Beginning around 2009, smartphone use skyrocketed, which researchers believe may be responsible for the 17 per cent bump between 2009 and 2015 in the number of students sleeping seven hours or less.

Not only might teens be using their phones when they would otherwise be sleeping, but previous research suggests the light wavelengths emitted by smartphones and tablets can interfere with the body's natural sleep-wake rhythm.

"Teens sleep began to shorten just as the majority started using smartphones," said Jean Twenge, from San Diego State University in the US.

Students might compensate for that lack of sleep by dozing off during daytime hours, said Zlatan Krizan from Iowa State University.

"Our body is going to try to meet its sleep needs, which means sleep is going to interfere or shove its nose in other spheres of our lives," Krizan said.

"Teens may catch up with naps on the weekend or they may start falling asleep at school," he said.

For many, smartphones and tablets are an indispensable part of everyday life, so they key is moderation, Twenge said.

Limiting usage to two hours a day should leave enough time for proper sleep, she said.

"Given the importance of sleep for both physical and mental health, both teens and adults should consider whether their smartphone use is interfering with their sleep," she said.

"Its particularly important not to use screen devices right before bed, as they might interfere with falling asleep," she added.