Lack of sleep blights children's education
Sleep deprivation is a significant hidden factor in lowering the achievement of school pupils, according to researchers carrying out international education tests.
It is a particular problem in more affluent countries, with sleep experts linking it to the use of mobile phones and computers in bedrooms late at night.
It is such a serious disruption that lessons have to be pitched at a lower level to accommodate the sleep-starved learners, the study found.
The international comparison, carried out by Boston College, found the United States to have the highest number of sleep deprived students, with 73% of 9 and 10 year olds and 80% of 13 and 14 year olds identified by their teachers as being adversely affected.
The BBC's Jane O'Brien reports.