Even an hour of night-time light exposure from tablets or smartphones can significantly disrupt sleep in teenagers, a new study has warned.

The research found that the sleep biology of boys and girls aged 9 to 15 who were in the earlier stages of puberty were especially sensitive to light at night compared to older teens.

In lab experiments, an hour of nighttime light exposure suppressed their production of the sleep-timing hormone melatonin significantly more than the same light exposure did for teens aged 11 to 16 who were farther into puberty.

The brighter the light in the experiments, the more melatonin was suppressed. Among 38 children in early to middle puberty an hour of 15 lux of light (dim “mood” lighting) suppressed melatonin by 9.2 per cent, 150 lux (normal room light) reduced it by 26 per cent, and 500 lux (as bright as in a supermarket) reduced it by 36.9 per cent.

The 29 teens in the late or post-puberty stage were also affected, but not as much. Exposure to 15 lux did not suppress melatonin at all, 150 lux reduced it 12.5 per cent, and 500 lux reduced it by 23.9 per cent.

The effects were the same for boys and girls, researchers said.

“Small amounts of light at night, such as light from screens, can be enough to affect sleep patterns,” said study senior author Mary Carskadon, professor of psychiatry and human behaviour in the Alpert Medical School of Brown University.

“Students who have tablets or TVs or computers – even an ‘old-school’ flashlight under the covers to read – are pushing their circadian clocks to a later timing. This makes it harder to go to sleep and wake up at times early the next morning for school,” said Carskadon.

Carskadon, lead author Stephanie Crowley of Rush University Medical Centre, and their co-authors said children and their parents should limit use of screens at bedtime, even though it has become pervasive.

One study found that 96 per cent of teens use at least one form of technology in the hour before going to bed.

The study was published in the Journal of Clinical Endocrinology & Metabolism.