Children who sleep in the same room as their smartphone may be getting less sleep every night, a new study has found.

The US study involved over 2,000 children aged nine and 12, the majority of whom had some sort of small screen, such as a smartphone, by their bed at night.

The researchers found that those who slept with smartphones in their room got almost 21 minutes less sleep every night compared to children who did not have these devices in their room.

It is widely believed that the light emitted from these devices can affect the body's circadian rhythm (internal clock). The brain uses the amount of light to determine when to produce more of the hormone, melatonin, which brings on sleepiness.

Previous research has found a similar link between reduced sleep and having a TV in the bedroom. However, the researchers believe that a smartphone may reduce the amount of sleep a child gets even more than a television because smartphones tend to be held closer to the face.

The light from these phones therefore may delay the release of melatonin more than the light from a television set, they suggested.

Meanwhile, the study also found that those with smartphones in their bedrooms also reported less regular sleep, possibly due to their phone alerting them to various incoming messages.

The researchers from the University of California at Berkeley noted that shorter sleep duration and poorer quality sleep can have a big impact on areas such as behaviour and academic performance. They have also been linked to weight gain.

They said that more research is needed to see if smartphones are the actual cause of reduced sleep, but added in the meantime, parents should keep a close eye on how much time their children are spending on these devices.

Details of these findings are published in the journal, Pediatrics.