Children are becoming increasingly anxious due to too much time in front of TV and computer screens.
Inactive lifestyles are also to blame for the negative impact on children's wellbeing, according to a British government briefing document from Public Health England.
It said higher levels of TV viewing were having a negative effect on children's wellbeing, including lower self-worth, lower self-esteem and lower levels of self-reported happiness.
Children who spent more time on computers, watching TV and playing video games also tended to experience higher levels of emotional distress, anxiety and depression, it said.
Last week, a study found half of all seven-year-olds do not get enough exercise - and girls are far less active than boys.
Only 51 per cent of all seven-year-olds in the UK achieve the recommended hour of exercise every day, with the figure being just 38 per cent in girls compared with 63 per cent in boys.
Half of this age group is also sedentary for an average of 6.4 hours or more every day, experts found.
The research, published in the online journal BMJ Open, found that children of Indian origin and those living in Northern Ireland were among the least physically active of all seven-year-olds.
The briefing paper was released to back a Public Health England campaign urging parents to rethink their children's eating and exercise habits ahead of the new school term.
The Smart Restart campaign includes tips to reduce time in front of TV, encourage walking to school and for pupils to have a healthy lunch.