Teenagers are not known for their inspirational sleep habits (think constantly pushing snooze to avoid going to school or staying up late doing whatever it is kids these days do). But a new study published in the journal Sleep Medicine suggests they’re getting less sleep today compared to teens were just a few years ago, and smartphones and tablets might be to blame.

The research comes from Jean M. Twenge, a professor of psychology at San Diego State University, and collaborators from Iowa State University. Twenge is a leading voice on how screens are affecting younger generations—she wrote the book iGen: Why Today’s Super-Connected Kids Are Growing Up Less Rebellious, More Tolerant, Less Happy–And Completely Unprepared for Adulthood.

Twenge and her team looked at the data from two nationally representative surveys gathered between 2009 and 2015, totaling more than 360,000 teenagers in the U.S. Teens in grades 8 through 12 answered questions about how much sleep they got on school nights, how frequently they got 7 hours of sleep (which is interesting to note, because most experts agree that teens need 9 hours of sleep) and questions about their use of devices.

The researchers found that in 2015, about 40 percent of teens were sleeping less than 7 hours most nights. Compared to 2009, that’s around a 17 percent increase in the amount of teens not getting enough rest. Those who spent 5 hours online daily were 50 percent more likely to get insufficient sleep compared to peers who only went online for an hour a day. Perhaps even more telling is how much more sleep-deprived teens are today compared to a few decades ago. The amount of teens getting less than 7 hours of sleep in 2015 is a whopping 58 percent more than teens in 1991.

The researchers looked at a variety of potential reasons for the spike in sleeplessness, including watching television, staying up to do homework or working for pay. But they found those factors didn’t change much during that 6-year period, while on the other hand, use of social media and mobile devices increased between 2009 and 2015. By 2011 “around 72 percent of adolescents reported using a cell phone in their bedroom in the hour before bedtime.” 

“Teens’ sleep began to shorten just as the majority started using smartphones,” Twenge said in the study’s press release. “It’s a very suspicious pattern.”

There are a few ways that smartphones (and devices in general) can affect sleep. They keep you awake in the literal sense, where you’re actually using them to, for example, scroll through social media well into the night. But our devices also confuse our brains due to the type of light they emit. That blue light that comes from their screens is the same sort of light we get during the day, and it signals our brain to stay up when we’re exposed to it at night.

Being chronically sleep deprived is bad for everybody, and has been linked to a wide range of negative outcomes like obesity and diabetes to psychological impacts like depression and substance abuse, according to the study.

But it’s particularly concerning for adolescents who need more sleep than adults (who need 7 to 9 hours, according to the National Sleep Foundation), not to mention the fact that the way devices may affect their growing brains is still unclear, something Twenge has expressed concern about before. This is also linked to the ongoing debate about early school start times, which many experts believe need to be changed. “Sleep-deprived teenagers are disproportionately likely to be involved in motor vehicle crashes,” senior behavioral and social scientist at RAND Corporation Wendy Troxel, PhD, writes for Thrive Global, adding that they’re also more likely to “abuse drugs, alcohol, and cigarettes.”

“Given the importance of sleep for both physical and mental health,” Twenge said in the press release, “both teens and adults should consider whether their smartphone use is interfering with their sleep.”

Read the study here and the press release here