At least a quarter of GCSE students are losing sleep, with many suffering stress over tests and coursework, a survey suggests.
Parents said exams were keeping children up at night
A poll of parents in England and Wales found 26% of 14 to 16-year-olds had missed sleep in the previous month.
Some 17% had done so because of school-related concerns, the survey for the Times Educational Supplement found.
Overall, 21% of the 500 parents of four to 16-year-olds interviewed said their children had missed sleep.
The biggest cause among the youngest age group was nightmares, while for the oldest it was academic pressures.
Only 6% said their children's school recommended how much sleep was needed, with 3% suggesting bedtimes.
The survey also found 68% of children had televisions in their bedrooms, with 34% allowed to watch after going to bed.
Some looked at more than two hours of programmes before going to sleep.
Meanwhile, 12% had internet access in their bedrooms.
The survey found that 99% were in bed by 11pm and 97% had switched off their lights by 11.30pm.
Commenting on tests stress, the Department for Education and Skills said: "Children sit just three sets of national curriculum tests during their 11 years at school.
"Teachers are well accustomed to ensuring that their children know what to expect and can cope well with the tests."