Blue light emitted by smartphone screens disrupts sleep in children and teenagers, according to a study which found that young people are more vulnerable to the negative effects of the devices than the adults.
Researchers noted that the brains, sleep patterns, and eyes are still developing in children and teenagers.
Of more than five dozen studies looking at youths aged five to 17 from around the world, 90 per cent have found that more screen time is associated with delayed bedtimes, fewer hours of sleep and poorer sleep quality, researchers said.
Since their eyes are not fully developed, children are more sensitive than adults to the impact of light on the internal body clock, they said.
"The vast majority of studies find that kids and teens who consume more screen-based media are more likely to experience sleep disruption," said Monique LeBourgeois, associate professor at the University of Colorado Boulder in the US.
"We wanted to go one step further by reviewing the studies that also point to the reasons why digital media adversely affects sleep," said LeBourgeois.
When light hits the retina in the eye in the evening hours it suppresses the sleep-promoting hormone melatonin, delaying sleepiness and pushing back the timing of the body clock.
"We know younger individuals have larger pupils, and their lenses are more transparent, so their exposure and sensitivity to that light is even greater than in older individuals," he said.
Previous research has shown that when adults and school- age children were exposed to the same amount and intensity of light, the children's melatonin levels fell twice as much.
Studies have also shown that short-wavelength "blue light" - ubiquitous in hand-held electronics - is particularly potent at suppressing melatonin.
"Through the young eyes of a child, exposure to a bright blue screen in the hours before bedtime is the perfect storm for both sleep and circadian disruption," LeBourgeois said.
The "psychological stimulation" of digital media - whether it's exposure to violent media or texting with friends - can also sabotage sleep by boosting cognitive arousal.
Children and adolescents who leave a phone or computer on overnight in their bedroom are significantly more likely to have trouble sleeping.
More than 75 per cent of youths have screen-based media in their bedrooms, 60 per cent interact with them in the hour before bedtime, and 45 per cent use their phones as an alarm.
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