Young people who text in the dark get less sleep and perform worse at school than those who put down their phones when the lights go off. Not entirely surprising, but, in contrast, texting before lights out was found to have no impact on academic performance.

Research suggests that the sleep-delaying impact of “blue light” emitted from phones is intensified in the dark, resulting in students feeling sleepier during the day and performing worse in tests.

The research, published in the Journal of Child Neurology, is the first of its kind to link the nighttime texting habits of American teenagers to school performance.

It adds to the growing body of evidence on the importance of sleep for teenage brains, suggesting it should be given the same weight as nutrition and exercise in promoting healthy bodies and minds.

The research also found that children aged five to 16 spend more than six hours a day in front of a screen, well above the two hour recommendation of the American Academy of Pediatrics.

Author Xue Ming, professor of neuroscience and neurology at Rutgers New Jersey Medical School, says the study highlights the effects of smartphone use on teenage brains.

“We need to be aware that teenagers are using electronic devices excessively and have a unique physiology,” says Ming. “They tend to go to sleep late and get up late. When we go against that natural rhythm, students become less efficient.”

The effects of “blue light” emitted from smartphones and tablets are intensified when viewed in a dark room, Ming says. This short wavelength light can have a strong impact on daytime sleepiness symptoms since it can delay melatonin release, making it more difficult to fall asleep - even when seen through closed eyelids.

“When we turn the lights off, it should be to make a gradual transition from wakefulness to sleep,” Ming says.

“If a person keeps getting text messages with alerts and light emission, that also can disrupt his circadian rhythm. Rapid Eye Movement sleep is the period during sleep most important to learning, memory consolidation and social adjustment in adolescents. When falling asleep is delayed but rising time is not, REM sleep will be cut short, which can affect learning and memory.”

Ming suggests that educators recognise the sleep needs of teenagers and incorporate sleep education in their curriculum.

“Sleep is not a luxury; it’s a biological necessity. Adolescents are not receiving the optimal amount of sleep; they should be getting eight-and-a-half hours a night,” says Ming.

“Sleep deprivation is a strong argument in favor of later start times for high schools.”