How does screen time affect a teenager's sleep?

What the evidence says

Screens and teen sleep are closely linked, but the reasons are a bit more complicated than just "blue light is bad." Studies following thousands of teenagers over time show that more evening screen use goes along with going to bed later, sleeping fewer hours, and feeling more tired during the day.

There are a few ways screens might cause this. First, time spent on a phone or game is time not spent sleeping — this is called displacement. Second, texting, social media, and games are stimulating and can keep the brain alert, making it harder to wind down. Third, the light from screens, especially blue light, can suppress melatonin, the hormone that signals it's time to sleep, and shift the body's internal clock later.

Interestingly, the melatonin part may matter less for teenagers than for younger kids or adults. One careful lab study found that turning down evening light exposure barely changed teens' sleep, and a review noted that children seem more sensitive to light-related melatonin suppression than teens or adults are. This suggests that for teenagers, screens may mess with sleep mainly by stealing time and revving up the brain, not just through light.

A real-world experiment backs this up: when researchers had students stop using screens after 9pm for two weeks, the students fell asleep earlier, slept longer, and felt more alert the next day. This kind of before-and-after study is stronger evidence than just watching who naturally uses more screens.

Because a lot of this research is observational, it cannot fully prove screens directly cause poor sleep — other things like anxiety, homework load, or family rules could play a role too. Still, the overall pattern is consistent enough that Canadian and international health groups recommend keeping screens out of the bedroom and stopping use at least an hour before bedtime.