One in five schoolchildren wakes up in the night to look at social media, a new study has warned.
Academics in Cardiff found that youngsters aged between 12 and 15 will get up at least once a week on a school night to check sites such as Twitter and Facebook.
And, unsurprisingly, more than half of those reaching for their smartphones when they should be fast asleep said they were "almost always" tired in classes the next day.
Dr Kimberley Horton, from the Wales Institute of Social and Economic Research Data (Wiserd), said the study showed that it was important for parents to discourage adolescents from using social media during the night.
She said: "Having a regular waketime and using social media during the night appear to be more important in determining whether a young person is always tired during the day than the time they go to bed, how long they spend in bed and having a regular bedtime.
"No amount of effort to develop regular bedtimes or to lengthen the time in bed would seem to be able to compensate for the disruption that this can cause."
The Wiserd researchers asked 848 school pupils in Wales how often they woke at night to use social media - with 22% of Year 8 pupils and 23% of those in Year 10 answering 'almost always'.
A further a 14% of the younger students and 15% of the older group said they did so at least once a week.
A spokesman said: "Those surveyed were also asked how often they felt tired at school. More than half of those who reported 'almost always' waking to use social media also said they 'almost always' go to school feeling tired.
"This was much higher than the overall percentage of respondents."
The study also found "substantial" proportions of pupils going to bed very late - with more than a quarter of Year 10 pupils going to sleep at midnight or later on a school night.
The new research comes after fresh debate on whether school start times should be put back later to combat sleep-deprivation among pupils.
However, the Wiserd paper argues that children would be less likely to have regular waking times as a result - and it was regular routines which would be likely to make children feel less tired.
It said: "Having a regular morning routine may actually prove to be a very important feature in helping adolescents concentrate and enjoy their learning, something that may actually be undermined by changes to the school day."
The study entitled Routines And Rest: The Sleep Behaviours Of 12 To 15-Year-Olds - will be presented at the British Educational Research Association in Queen's University, Belfast, on Wednesday September 16.