New data, from the Scottish Government's long-term Growing Up in Scotland project, found that on an average school day, 34 per cent of 12-year-olds were online for up to three hours messaging people via text, Instagram, Snapchat or playing online games.
A further 14 per cent spent between three or five hours glued to their screens every weekday, while 13 per cent spent more than five hours online.
The data also revealed that the children who spent just a limited amount of time on social media said they were more satisfied with their lives than those who either spent no time or excessive periods of time, on it.
Those who were online for between 30 minutes and two hours reported the highest scores, while children who spent seven or more hours daily reported the lowest average score. Those who spent no time at all on social media also had lower than average life satisfaction.
Boys also spent less time using social media than girls - 14 per cent said they did not spend any time on it during Mondays to Fridays, compared to 15 per cent of girls who said they could spend five or more hours messaging people.
The new data comes after a Scottish Government report found there was "contradictory evidence" about the effect of social media use on the mental health and well-being of teenagers. It said that use of social media could "promote social connectedness and self-expression and provide emotional support and access to expert advice" but also be "a source of cyber bullying, isolation and unfavourable social comparisons".
The Growing Up in Scotland study, which tracks the lives of thousands of children born in Scotland between June 2004 and May 2005, asked 3419 12-year-olds about their social media and internet habits, as well as bullying, weight and whether they had smoked or tried alcohol.
The researchers found that one in five had tried alcohol, though only one per cent claimed to have been drunk. Four per cent had tried a cigarette - with seven per cent saying they had tried vaping.
The data did find that children living in more deprived areas were more likely to have tried cigarettes than those in the least deprived ones. And it said there was "a clear relationship between trying a cigarette and trying an e-cigarette - 39 per cent who had tried vaping had also tried a cigarette, with 75 per cent of children who had experimented with smoking, also trying e-cigarettes.
And children who had smoked were significantly more likely than those who had not to have also drunk alcohol. Seventy per cent of those who said they had tried a cigarette had also drunk alcohol.
In terms of anti-social behaviour, from a list of nine examples from shoplifting to vandalism, graffiti to fighting, 30 per cent of children reported being involved in at least one of the activities, with boys more likely than girls to have done so - 40 per cent compared with 21 per cent. The most common anti-social behaviour was fighting, with 19 per cent of children saying they had done this, 29 per cent of boys compared to nine per cent of girls.
Bullying was a relatively common experience with a significant minority of children experiencing it in some form on a regular basis, and the most common form was being called names. Forty three per cent said they had ever experienced this - with ten per cent saying it they were "made fun of" most days.
Girls were more likely than boys to be picked on by being left out of games and chats, while boys were more likely than girls to get picked on by shoving, pushing or fighting. And despite the social media use, the vast majority of children - 86 per cent - had never been picked on via messages or online posts, though this form of bullying was more common for girls than boys - 17 per cent compared with 12 per cent.
There was also a disconnect between children and their parents about what a health weight is and looks like. A total of 62 per cent of children who were overweight perceived themselves to be about the right size and 86 per cent of parents whose children were overweight perceived their child’s weight as normal.
A further 32 per cent of children who were obese perceived themselves to be about the right size and 41 per cent of parents whose children were obese perceived their child’s weight as normal.
Relationships between children and their parents who did not live with them were noticeably weaker than those with resident parents, with just half of the children feeling they could always count on them if they had a problem.
Although similar proportions of boys and girls said this was always true, boys were more likely than girls to say it was often true - 25 per cent compared with 16 per cent - and the researchers said the results "very tentatively suggest that boys with a non-resident parent tend to have a better relationship with that parent than girls do."