Getting enough sleep has long been an issue for teenagers - and their parents - but concerns are growing that the use of electronic devices is increasing sleep deprivation among young people.
Jane Ansell, director of the charity Sleep Scotland, which has been running projects in a number of schools, said that more than half of teenagers do not get enough sleep.
She started researching the issue after realising that some of the children being referred to her did not have attention deficit hyperactivity disorder, as was thought, but were sleep deprived.
Hormonal changes affect teenagers' body clocks, meaning that they go to sleep and wake up later.
Some schools in England have introduced a later starting time of 10am to fit in with adolescent sleep patterns. Monkseaton High School in North Tyneside, England, was the first to trial a later start time in 2009, but has since reverted to 8.50am.
The UCL Academy, which opened in north-west London in September 2012, has changed its start time to 10am for sixth-form students.
73% of adolescents who told a US poll that they felt unhappy, sad or depressed also reported not getting enough sleep at night and being excessively sleepy during the day
Source: US National Sleep Foundation survey (2006). bit.lyTeensAndSleep
Lack of sleep and its effects on students
Percentage of 13- and 14-year-old students whose lessons are affected by lack of sleep, according to their teachers
Source: Trends in International Maths and Science Study (2011), exhibit 8.22. bit.lyLackSleep
Norway 67%US 78%
Hong Kong 73%
Saudi Arabia 72%
New Zealand 62%