Sleep deprivation can start very early in life. Between televisions, tablets, computers and cell phones, most kids have a screen in their rooms. And watching those screens can interfere with sleep.

“Parents assumed that TV was helping their kids wind down,” said Rebecca Spencer, the co-author of a study on the role TV plays in young children's sleep. “But it didn't work. Those kids weren't getting good sleep, and it wasn't helping them fall asleep better.” Spencer has been studying the effects of sleep, including the importance of naps for young children, for more than a decade.

Children who had a TV in their bedroom watched TV later at night, watched more adult TV programs and displayed more negative emotions.

The University of Massachusetts, Amherst researchers found that children who watched more TV got significantly less and lower quality sleep. On average, young children without a TV in their bedrooms slept 30 minutes more at night than those with a TV in their bedroom — three and a half hours more each week.

Kids with a television in their room did try to make up for lost sleep by napping more during the day, but this amounted to less than half of the sleep time they had lost at night.

The study involved a diverse group of 470 preschoolers from Western Massachusetts. Each child wore a wrist monitor for 16 days.

Parents tend to overestimate how long their children sleep, so the use of wrist monitors is one of this study's strengths, according to Spencer. Previous research has mostly relied on self-reported information on sleep length and quality.

Children who had a TV in their bedroom watched TV later at night, watched more adult TV programs and displayed more negative affect — more negative emotions and poorer self-concept — than children who slept in a TV-free bedroom.

Over a third of the children had a bedroom TV, and a third of this group fell asleep with the TV on, often watching adult programming.

Teens' sleep problems are well-documented, including a school day that many think starts too early.

“The good news is, this is addressable,” says Spencer, but it’s up to the parents to confront the problem. The current study focused on televison, but Spencer has plans for more studies that will examine the impact of hand-held digital devices, such as iPads and smartphones on children's sleep.

The American Academy of Pediatrics recommends that daily screen time for 2- to 5-year-olds be limited to one hour of “high-quality programs,” and that parents should watch the programs with their children.

The study appears in Sleep Health.