The last thing my 11-year-old does before she goes to sleep is put her iPod on the nightstand. And that could mean less sleep for her, researchers say.

There's plenty of evidence that children who have televisions in their rooms get less sleep. This is one of the first studies to look at whether having a small screen like an iPod or smartphone in the room also affects rest.

The study, which was published Monday in Pediatrics, looked at 2,048 racially diverse fourth-graders and seventh-graders who were participating in a study on childhood obesity in Massachusetts. Lack of sleep is considered a risk factor for obesity, so the children were asked how long they slept and if they felt they needed more sleep.

They also were asked how often they slept with an iPod, smartphone or cellphone in their bed or next to the bed. More than half of the children, 57 percent, said they slept near a small screen.

Those children reported getting 20.6 fewer minutes of sleep on weekdays, compared to children who didn't have the devices in the bedroom. Those children were also more likely to say they felt like they hadn't gotten enough sleep.

The study also looked at TVs in the bedroom and found that children who slept in a room with a TV reported 18 fewer minutes of sleep than those without a TV, on par with other studies. And the big screens were even more common than the small screens — three-quarters of the children said they had a TV in their room. But they were less likely to feel like they missed out on sleep than the kids with small screens.

With both the TVs and the small screens, children went to sleep later, the researchers say. The small-screen sleepers hit the hay 37 minutes later than their screenless peers, and TV-watchers went to bed 31 minutes later. All the children were getting up at the same time because they had to get off to school.

And here's one more wrinkle: Children who said they played video games or watched DVDs during the day also said they felt less rested. But the negative impact was much smaller than for small screens or TVs in the bedroom.

This study wasn't designed in a way that could figure out what was causing the sleep loss and tiredness — whether the kids were actually using the devices thus exposing themselves to light and stimulating content, say, or whether getting calls or alerts during the night interrupted sleep.

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