Results showed that stress was the most significant factor contributing to sleeping problems.

Students who performed poorly academically reported that they fell asleep later and slept less on school nights than those with better grades.

According to Professor Chung, there are five factors which affect sleep among adolescents: the time they go to bed; the amount of pressure they are under; whether they drink alcohol; their school year; and their age.

'The number of hours of sleep is not the most important factor - it's the quality that counts. A person suffering from insomnia might lie in bed for more than 30 minutes and still not fall asleep, or they

might wake up in the middle of the night and stay awake for over half an hour,' says Professor Chung.

He adds that insomnia affects one's concentration - the person will feel tired during the day and might suffer from the condition for more than a month.

One 14-year-old girl who suffers from insomnia (she wishes to remain anonymous) says she has not had any quality sleep over the past year and it has become worse since she started her Year Nine studies.

Sometimes she goes to bed at 10pm but cannot fall asleep. Some nights she lies awake until 4am. 'I feel tired in the morning when I wake up. It's fine during the day, but in the evening when I get home, my eyes get heavy again. But I can't fall asleep at night, every night, and the cycle repeats itself,' she says.

Professor Albert Li from the department of paediatrics at Chinese University of Hong Kong says: 'School work, the living environment, whether the person is sharing the bedroom with someone else and noise from the television - all of these affect sleep.

'Good sleep hygiene [guidelines] is very important and it should be introduced to children at a young age,' he says. 'There are no fixed rules on how to get good sleep. Do whatever that relaxes you.'