In the study, when there was a TV or video games in the bedroom, children spent less time reading, sleeping or participating in other activities, which had a ripple effect on several outcomes.
As a result, these children did not do as well in school and were at greater risk for obesity and video game addiction, Prof Gentile said.
Researchers were able to track these effects over a period of six months to two years.
The study, published in the journal Developmental Psychology, also found children with bedroom media watched programs and played video games that were more violent, which increased levels of physical aggression.
Prof Gentile said it stands to reason that most parents are not fully aware of what is happening behind closed doors.
"When most children turn on the TV alone in their bedroom, they are probably not watching educational shows or playing educational games," Prof Gentile said.
"Putting a TV in the bedroom gives children 24-hour access and privatises it in a sense, so as a parent you monitor less and control their use of it less," he said.
Having bedroom media significantly changes the amount of time children spend with media, changes the content they view, but also changes what children do not do, such as reading.
National studies show that more than 40 per cent of children, ages 4-6, have a TV in their bedroom, and a substantial majority of children 8 and older have a TV or video game console in their bedrooms.
While this study looked specifically at TVs and video games in the bedroom, Prof Gentile expects the effects to be the same, if not stronger, given the access children now have to digital devices.
It may be natural for parents to wonder why a TV in the bedroom is any different from any other room in the home, Prof Gentile said.
There is no direct link between the physical presence of a TV and poor grades. However, bedroom media makes it easier for children to spend more time watching or playing, which displaces other beneficial and healthful activities.
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