Social media can make children more likely to have eating disorders or try drugs because it makes them more impulsive, a new study suggests.
Too much social media - which includes Facebook, Twitter, WhatsApp, YouTube, Instagram and SnapChat - makes them more impulsive, often leading to bad decisions that will affect them for the rest of their lives, warn scientists.
Children spending a significant amount of time looking at screens, and not getting enough sleep, are most vulnerable, the study found.
Lead author Dr Michelle Guerrero, a paediatrican at the University of Ottawa, said: "Impulsive behaviour is associated with numerous mental health and addiction problems, including eating disorders, behavioural addictions and substance abuse."
Her team analysed more than 4,500 eight to 11 year olds. Those who used smartphones more than two hours a day and slept less than nine hours a night were more impulsive.
The results were based on the participants' self reported scores on eight traits such as perserverance, seeking out thrills, setting goals, being sensitive and acting rashly.
Dr Guerrero said the findings reinforces the importance of the Canadian 24-Hour Movement Guidelines for Children and Youth.
These advise five to 17 year olds get at least 60 minutes a day moderate to vigorous exercise, have a maximum two hours of recreational screen time and manage nine to 11 hours uninterrupted sleep every night.
Meeting all three pillars of the recommendations was associated with more favourable outcomes.
Dr Guerrero said: "Impulsive behaviour is associated with numerous mental health and addiction problems, including eating disorders, behavioural addictions and substance abuse.
"This study shows the importance of especially paying attention to sleep and recreational screen time.
"When kids follow these recommendations, they are more likely to make better decisions and act less rashly than those who do not meet the guidelines."
In one of the first studies of its kind her team looked specifically at the link between screen time and behaviour in adolescents.
Dr Guerrero said: "We were looking at how excessive screen time leads to impulsive behaviour."
Impulsivity was defined as acting "without realising the risk involved or the consequences involved with the behaviour."
The characteristic is often connected to mental health issues and is a key component of attention deficit hyperactivity disorder.
Dr Guerrero said: "When we don't think before we act, it has huge impacts on our lives."
A key discovery was how easily preventable it is, as as it involves a minor life adjustment - including less screen time and more sleep.
Dr Guerrero said: "The behaviours we looked at in our study are modifiable. They can be changed right now, today."
It adds to increasing evidence that screens are harming children's mental health, she said.
Exercise was also found to lead to less impulsive youngsters. What is more, the benefit of a full night's sleep is a major factor.
Dr Guerrero said: "When people don't sleep long enough, their behaviour is more impulsive."
The research is the first set of data from the Adolescent Brain Cognitive Development Study, which plans to follow the participants for 10 years.
Future feedback will shed fresh light on how physical activity, screen time and sleep relate to children's impulsivity, Dr Guerrero added.