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People who spend more time staring at their phones before bedtime get a worse night’s sleep, researchers found, in the first study that directly measures screen time using a mobile application.

Why it matters:

Previous studies have found an association between using devices with screens and poor sleep quality. However, scientists aren’t sure exactly why that is. Exposure to blue-colored light, such as that produced by electronic screens, is known to stimulate wakefulness. Electronic device use before bedtime might also simply stress us out. And some research has shown that there are associations between simply having electronic devices in the bedroom and worse sleep — possibly pointing toward some other confounding factor.


The nitty-gritty:

Dr. Gregory Marcus, cardiologist at the University of California, San Francisco, used data from the Health eHeart Study, which was set up to do research on cardiovascular disease. Participants in the study recorded many different things about themselves, including when they went to bed, how long they slept, and how well they slept, but didn’t know when reporting the data that it might be examined for a relationship with cellphone use.

Then, Marcus and his team invited those same people to download a smartphone app to track when their phones’ screens were on. One hundred and thirty-six people used the app for at least a month and also had sleep quality data on file — and, among them, Marcus found that more screen time was associated with poorer quality sleep. Specifically, using a smartphone closer to bed was more strongly associated with poorer sleep quality than using a smartphone earlier in the day. The study was published Wednesday in PLOS ONE.

But keep in mind:

Mark Rea, director of the Lighting Research Center at Rensselaer Polytechnic Institute, who was not involved in this study, said that there isn’t enough data there to say why people who spent more time with smartphones had worse sleep.


Thinking back to the recent World Series, for instance, “it’s possible that if you are a Cubs fan or an Indians fan, and you’re looking at your phone, it really has nothing to do with the light [impacting your circadian systems],” Rea said.

Likewise it’s also possible that poor sleep could lead to more screen time. The study just correlated the two and didn’t show causation. Finally, since the study used data from two different time periods, it wasn’t able to determine whether phone use on a particular day impacted the quality of sleep that night.

What they’re saying:

As the first sleep-related study to measure smartphone use with a mobile app, the study should be an example for other screen-time studies, said Lauren Hale, a board member of the National Sleep Foundation.

“There’s more to be done,” Hale said. “This is only focusing on one tiny aspect of our intimate relationship with digital media.”

The bottom line:

Smartphones are but one aspect of a modern society filled with constant stimulation, and researchers aren’t surprised that they’re associated with lower quality sleep.