An hour of screen time immediately before bed is fine for most teenagers, according to research by an Australian university.
But two hours is too much and likely to disrupt their sleep, says Associate Professor Michael Gradisar, a clinical psychologist at Flinders University in Adelaide.
He says a review of four years of studies at the university and around the world shows moderate technology use isn't as harmful for adolescent sleep as many health professionals and parents believe.
"Experiments with video games show that if you give an experienced gamer a violent game before bed, their heart rate is not elevated and they sleep quite fine."
It's like they've adjusted to using technology, he says.
"This evidence challenges the idea that sleep problems in young people are because of technology."
A more common reason is anxiety or "body clock mistiming", which is similar to what happens when a person has jet lag.
"For the majority of the population, an hour of technology use before bed seems fine. But anything longer than two hours appears to be detrimental to sleep."
However, the Australasian Sleep Association says it is still best for teenagers to avoid technology for at least an hour before bed.
"Sleep is a complex biological process and further research is needed to translate the findings of this research into a recommendation," says spokesperson Dr Sadasivam Suresh, a paediatric sleep specialist.
"This research is valuable in adding information on actual sleep patterns in modern society. It is good science that helps us to understand sleep and health better."