Teenagers should not start school before 8.30am because tiredness is linked to car crashes, suicide, depression and poor performance, experts have said.
In a position statement published in a leading journal, the American Academy of Sleep Medicine warned that early lessons caused sleep deprivation for pupils which had a huge impact on their lives.
They said later start times optimised daytime alertness, reduced tardiness and improved school attendance.
"Early school start times make it difficult for adolescents to get sufficient sleep on school nights, and chronic sleep loss among teens is associated with a host of problems, including poor school performance, increased depressive symptoms, and motor vehicle accidents," said lead author and AASM Past President Dr Nathaniel Watson.
"Starting school at 8:30 a.m. or later gives teens a better opportunity to get the sufficient sleep they need to learn and function at their highest level."
Teenagers aged between 13 and 18 are advised to sleep between eight and 10 hours but most report sleeping seven hours or less on school nights.
Experts warned that school times work against adolescent circadian physiology, which can be nearly three hours behind that of an adult, leading to chronic sleep deprivation. A lack of sleep is known to lead to poor school performance, obesity, heart problems, metabolic disorders, suicidal thoughts, risk-taking behaviours and athletic injuries.
Although most schools in Britain start between 8.30am and 9am, British experts said that it would be helpful if they started even later.
Dr Paul Kelley, an expert in sleep, circadian and memory neuroscience at The Open University, said: “I actually think teenagers shouldn’t be starting school until 10am or later. Circadian rhythms shift dramatically during puberty and that impact continues right through university years.
“The lag peaks in males at around 19, so university students would benefit from starting at 11 or 1130. We’ve have shown that early start times are unfair for more than half of students. They are not at their best so there performance is always going to be sub-optimal.
“Sleep deprivation has effects right across all the body’s functions, it impacts all systems. But it is a tricky one to get right because everyone is different, so shifting times to help one group might be disadvantageous to another.”
Circadian rhythms change throughout life, with older people naturally rising far earlier than younger people. Dr Kelley believes that it is an evolutionary adaptation which ensures the safety of groups.
“My theory is that is a huge evolutionary advantage to have people awake and alert at different times of day,” he said.
In the US insufficient sleep also is associated with an increased risk of motor vehicle accidents, which account 73 per cent of deaths from unintentional injury in teenagers. Previous research has found that crash rates decline by 16.5 per cent when school start times are delayed by one hour.
Pilot programmes have also shown that delaying start times reduces daytime sleepiness, increases class engagement and reduces bouts of depression and irritability.
The position statement was published in the Journal of Clinical Sleep Medicine.