School break times have been getting shorter over the past two decades, as teachers try to pack more lessons into the day, a long-term study suggests.
Infants in England had 45 minutes less break time a week than in 1995, the University College London team found.
Secondary pupils had lost 65 minutes over the same period, they said.
The government said it had given schools the "autonomy" to make decisions about the structure and duration of their school day.
Pupils complained of fun activities being banned, not having enough time to eat their lunch, and missing their breaks due to others' poor behaviour.
Children and young people's social lives seem to have been curtailed as well, with fewer students than in 2006 reporting they had visited a friend's house after school, according to the research.
Playing video games and watching television had overtaken spending time with friends as the most common after school activity, the study found.
The researchers said this finding highlighted how "school is increasingly the main, and in some cases the only, context where young people get to socialise".
Researchers analysed questionnaires completed at 993 primaries and 199 secondaries in 2017 along with separate pupil surveys at 37 schools.
These were compared with surveys in similar schools in 2006 and 1995.
The team said their results gave the impression that breaks were being kept as "tightly managed and as short as possible" and this meant pupils could be missing out on social development.
Lead author Ed Baines, from UCL's Institute of Education, said: "Despite the length of the school day remaining much the same, break times are being squeezed even further, with potential serious implications for children's wellbeing and development.
"Not only are break times an opportunity for children to get physical exercise - an issue of particular concern given the rise in obesity - but they provide valuable time to make friends and to develop important social skills, experiences that are not necessarily learned or taught in formal lessons."
The researchers found what they described as a "virtual elimination" of afternoon breaks, with only 15% of infant pupils and just over half of juniors having one.
In 1995, 13% of secondary schools reported an afternoon break period but in 2017 only 1% said they had one.
Lunch breaks had also been cut down, the team said, with 82% of schools setting aside less than 55 minutes in 2017, compared with 30% in 1995.
Nearly 60% of schools also withheld breaks from children when they or their classmates had been poorly behaved or needed to complete work.
Head teachers' leader Geoff Barton said there was enormous value in unstructured free time for children to socialise and let off steam but schools had to balance this consideration against all the other demands expected of them.
The Association of School and College Leaders' general secretary said: "The fact is that school timetables are bursting at the seams because of the pressure to deliver a huge amount of learning and to prepare children for high-stakes tests and exams.
"It is therefore no surprise that school break times are shorter than they were 20 years ago.
"This may be regrettable but it is the result of a conscious decision by successive governments to expect more of schools."
A Department for Education spokesman said the government recognised the importance of physical activity in schools "to improve both physical and mental wellbeing".
"We are clear that pupils should be given an appropriate break and we expect school leaders to make sure this happens," he added.