Getting sufficient sleep is crucial for growing children, but new research suggests that the presence of a smartphone in the child's bedroom can negatively affect good sleeping habits.

For a new study, which was published in the journal Pediatrics on Jan. 5, Jennifer Falbe from the University of California Berkeley and colleagues looked at the sleeping habits of more than 2,000 boys and girls, who were in their fourth and seventh grade.

The researchers asked the participants about the electronic devices in their bedroom, the time they go to bed and wake up, and also how many days over the past week they felt that they needed more sleep.

The children who said they have a TV set in the bedroom said that they have 18 minutes less sleep during weeknights compared with those who do not have it in their room.

Those who slept near a smartphone, however, had nearly 21 minutes less sleep whether there was a TV set in their room.

Children who go to bed with a smartphone at hand also tend to have later bedtimes than those who have TVs in the bedrooms. The smartphone-toting kids, in particular, have 37 minutes later bedtime, which is six minutes later than the 31 minutes among those with TVs in their rooms.

Falbe and colleagues likewise found that the kids with a smartphone were more likely to feel they needed to have more sleep compared with the children who do not have a smartphone nearby during bedtime.

"Sleeping near a small screen, sleeping with a TV in the room, and more screen time were associated with shorter sleep durations," the researchers wrote. "Presence of a small screen, but not a TV, in the sleep environment and screen time were associated with perceived insufficient rest or sleep."

The researchers offered three possible explanations as to why devices with small screens are more disruptive for sleep compared with those that have large screens. One is that people tend to hold smaller devices near the face, which could delay the production of melatonin.

Watching TV is also a passive activity, while playing games or texting on a smartphone is more interactive. This causes increased cognitive and emotional arousal.

Small screen devices also have audible alerts for notifications, such as new SMS, which can wake children who are already asleep.

Preteens are recommended to have at least 10 hours of sleep every day, while teenagers have to have between nine and 10.

The researchers said that adopting technology rules, such as setting a curfew for using electronic devices, at home may help kids maintain healthier sleeping habits.

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