Gave your kids smartphones for the holidays? You might want to reconsider their bedtime.

Children who slept in the same room as small screens such as smartphones got almost 21 fewer minutes of shut-eye a night than those who didn’t, according to research published today by the journal Pediatrics. The two age groups studied — about 9 year olds and 12 year olds — also reported significantly less regular sleep with a phone nearby, possibly due to the buzz of text messages and app notifications.

The paper joins a growing body of research finding that gadgets correspond to shorter, more fitful nights, as well as other health issues such as weight gain. The evidence suggests that bright, blinking screens are affecting our biological timekeeper, the circadian clock.

While the Pediatrics study didn’t conclude that phones cause sleep deprivation, its lead author, Jennifer Falbe, said the case for clamping down on kids’ screen time is gaining strength.

Recent findings “caution against unrestricted access to media in children’s bedrooms,” Falbe, a researcher at the University of California, Berkeley, said in a phone interview. “The risks associated with shorter sleep duration and poorer sleep quality would include reduced academic performance, behavioral problems, possibly an increased risk for weight gain and possibly negative impacts on immunity.”

The human body uses light and darkness to influence circadian rhythms, the physical, mental and behavioral changes in a 24-hour cycle. The brain’s internal clock uses the amount of light to determine when to produce more melatonin, a hormone that brings on sleepiness.

The study in Pediatrics, funded by the U.S. Centers for Disease Control and Prevention and the National Institutes of Health, surveyed more than 2,000 fourth- and seventh-graders in Massachusetts as part of a broader investigation into obesity in children. The majority of kids surveyed slept with a small screen by their side.

The 21 minutes lost with smaller screens exceeded the 18 minutes given up by children who slept in a room with a TV. Since small devices are held closer to the face, they may delay the release of melatonin more than TV light, the researchers said.

TV and video games were also associated with less sleep. Seventy-five percent of kids in the study said they slept in a room with a TV. Each hour of TV- or DVD-viewing a day corresponded with almost four fewer minutes of sleep, compared with five fewer minutes for gamers.

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