Chronic fatigue syndrome is more common than previously thought, particularly among girls at the age of 16, research by the University of Bristol suggests.
Persistent exhaustion was found to affect 2% of 16-year-olds - and almost twice as many girls as boys.
Children from poorer families were also more likely to have the condition.
The Bristol study, published in Pediatrics, asked more than 5,700 parents and their children about their experiences of persistent exhaustion.
Chronic fatigue syndrome, also known as myalgic encephalomyelitis or ME, is a debilitating disease which has a major impact on the lives of those affected. It causes persistent fatigue that does not go away with rest or sleep.
This large population study found that one in 50 16-year-olds have CFS/ME lasting more than six months and nearly one in 33 have CFS/ME persisting for more than three months.
Yet only one in 1,000 are diagnosed with the condition.
Not yuppie flu
In the study, those with CFS/ME missed, on average, more than half a day of school every week.
Boys and girls appeared to be equally affected at the age of 13, but by the time they were 16, CFS/ME was more common in girls.
Children from families with poor housing, financial difficulties and a lack of support for their mother were more likely to report problems of severe fatigue.
The study authors said this dispelled the commonly held view that CFS/ME was a "middle-class" illness, or "yuppie flu".
The diagnoses of the condition in the study were based on responses to questionnaires sent to teenagers and their parents, and were not made by a doctor.
Dr Esther Crawley, senior report author and consultant paediatrician specialising in CFS/ME, said treatment for teenagers with the condition was effective but few had access to treatment in the UK.
"Children attending my specialist service only attend two days a week of school on average. This means that only the most severe cases are getting help.
"As paediatricians, we need to get better at identifying CFS/ME, particularly in those children from disadvantaged backgrounds who may be less able to access specialist care."
She said experts still did not know exactly why or how the condition is triggered, but research showed it could be successfully treated with cognitive behavioural therapy (CBT) in young people.
Sonya Chowdhury, chief executive of Action for ME, said the study pointed to the need for more effective treatments.
"We know from contact with the parents of children with ME that this disabling condition impacts on every area of family life.
"The reality is that many young people miss considerably more than half a day of school a week, while for the most severely affected, their disabling symptoms are compounded by the isolation and loss that comes with being housebound and/or bedbound."