Children who have access to tablets or smartphones in their bedrooms get less sleep than youngsters who do not have the devices with them at night, a US study has found.

The findings, announced in Monday's edition of the journal Pediatrics, show that having a small screen within reach was slightly worse than a television set when it came to sleep deprivation in a group of 2,000middle school pupils.

Overall, those with access to smartphones and tablets got nearly 21 fewer minutes of sleep per night than children whose rooms were free of such technology, and they were also more likely to say they felt sleep-deprived.

Those with a TV in the bedroom got 18 fewer minutes of sleep than children without televisions in their rooms.

The presence of a small screen, but not a TV, in the sleep environment, and screen time, were associated with perceived insufficient rest or sleep, according to the study led by Jennifer Falbe of the School of Public Health at the University of California, Berkeley.

These findings caution against unrestricted screen access in children's bedrooms, the study found.

Participants in the study included 2,048 fourth-and seventh-graders enrolled in the Massachusetts Childhood Obesity Research Demonstration Study in 2012 and 2013.