Kids who watch more TV or even have TVs in their bedrooms get less sleep, a U.S. study said Monday.

Researchers from MassGeneral Hospital for Children and Harvard School of Public Health found "a small but consistent association" between increased TV viewing and shorter sleep duration after studying more than 1,800 children aged from 6 months to nearly eight year.

The presence of a television in the room where a child sleeps was also associated with less sleep, particularly in minority children, the researchers said.

The study participants, children and their mothers, were enrolled in Project Viva, a long-term investigation of the health effects of several factors during pregnancy and after birth.

From the time the children were around six months old, and then on a yearly basis for the next seven years, mothers reported how much time each day infants were in a room where a TV was on, how much time older children watched TV daily, whether children aged four to seven slept in a room where a TV was present and their child's average daily amount of sleep.

The researchers found each additional hour of TV viewing was associated with seven fewer minutes of sleep daily, with the effects appearing to be stronger in boys than in girls.

For racial and ethnic minority children, the presence of a bedroom TV reduced average sleep around a half-hour per day.

The results support previous short-term studies finding that both TV viewing and sleeping in a room with a TV decrease total sleep time, which can have negative effects on both mental and physical health, the researchers said.

The findings were published in the U.S. journal Pediatrics.