Adrian Underwood, director of the Boarding Schools Association, said: "Schools have to be quite rigorous to make sure children get their eight hours' sleep a night.
"Although times differ slightly from school to school, lights out means lights out."
On average, pupils aged under 11 will be expected to be in bed before 9pm, said Mr Underwood.
Children aged 11 to 13 normally have "lights out" at 9.30pm. Those aged up to 16 are allowed to stay up until 10pm and sixth-formers until 10.30pm.
Almost 70,000 pupils board at private and state schools.
Mr Underwood said few schools stipulate sleeping time in contracts with parents, but that most parents expect schools to make sure that their children are in bed at a reasonable hour.
Life for pupils at Old Swinford Hospital, a state boarding school in Dudley, West Midlands, is typical of many. The 380 boarders are not allowed to have televisions in their rooms, although they are permitted to have have a computer, music equipment or a games console.
The youngest pupils are in dormitories of up to eight, and those over 16 are in single rooms.
Melvyn Roffe, the head, said: "There is a little flexibility surrounding when they go to bed - for example, if the older students are studying, they may be allowed to stay up until about midnight.
"You cannot force children to go to sleep, but we have found that generally there is so much packed into their day - they have a healthy diet and lots of exercise - that most sleep without much problem.
"If they are not sleeping, we have nursing staff, a school doctor and counsellor who can provide support, but it is rarely an issue.
"I have never had a conversation with a parent demanding to know why their child isn't getting eight hours' sleep a night."