Teens who use social media for three hours or more per day have problems falling asleep and then struggle to get up for school the next morning, experts say.
Their new study found that using sites such as Facebook and Twitter also led to frequent wakings and problems getting back to sleep.
Published in BMJ Open, the research involved 1,872 teenagers aged 13 to 15 who were asked about the times they went to sleep, whether they had trouble falling back asleep after waking in the night and what time they got up the next morning.
They were also asked how many hours per day they spent on social networking or messaging sites or apps such as Facebook, Twitter and WhatsApp.
Just over a third of the group (33.7%) were low users of social media (less than one hour per day), while 31.6% were average users (one to three hours a day), 13.9% were high users (three to five hours a day) and 20.8% were very high users (more than five hours a day).
The study found that heavy social media users were 23% more likely to fall asleep late on school nights (after 11pm) compared to average users, while very heavy users were more than twice as likely to fall asleep late.
Heavy users were also 56% more likely to sleep in on school days (after 8am) when they needed to be up, while very heavy users were 97% more likely.
Furthermore, very heavy users were 36% more likely to wake up throughout the night and have trouble getting back to sleep, while heavy users were 8% more likely.
A similar pattern occurred on free days such as weekends and holidays, with heavy and very heavy users more likely to stay up late and sleep in.
Low social media users were the least likely to fall asleep late – suggesting minimal social media use produces “optimal outcomes for sleep”, the researchers said.
They added: “These findings are consistent with the idea that social media displaces sleep: either directly or indirectly.”
The research also found that while girls reported more social media use, it was boys who were more likely to fall asleep late on free days and wake up late on school days.
In measures of poor sleep quality, 34% of boys typically took longer than 30 minutes to fall asleep and 21% reported difficulties falling asleep following night-time awakenings at least a good bit of the time.
The researchers, from the University of Glasgow, said going to sleep late on school days is of “particular concern” because late school day bedtimes “predict poorer academic and emotional outcomes”.
Dr Bernadka Dubicka, chairwoman of the child and adolescent faculty at the Royal College of Psychiatrists, said: “Lack of sleep can be hugely damaging, and is often related to poor mental health and academic achievement.
“Children and young people should also avoid using screens in the last hour before bed as this will help them sleep better at night.”
Dr Max Davie, officer for health improvement at the Royal College of Paediatrics and Child Health, said: “We recommend that young people stay off all screens for at least an hour before bed so that their brains have time to wind down.
“The content they are viewing plus the light emitted from screens can increase brain stimulation and make it difficult to fall asleep.
“While some screen manufacturers have introduced “night modes” claiming to emit less blue light, there is no evidence as yet that these are effective.
“Lack of sleep can have a significant negative impact not only on young people’s wellbeing, but on their relationships with family and friends and in terms of reaching their full potential at school.”
In January, MPs on the Commons science and technology committee said social media led to damaged sleep patterns, body image issues, bullying and online grooming.
And in February, the UK’s leading doctors, including the outgoing chief medical officer Professor Dame Sally Davies, said parents should not allow children to take phones and other electronic devices into their bedrooms or use them during mealtimes.