Teenagers have traditionally enjoyed a lie-in - but forcing them to get up early to go to school may actually be cruel, a neuroscientist has suggested.
Allowing teenagers a lie-in may improve results, research suggests
Oxford University professor Russell Foster said research shows young people perform better as the day progresses.
Exam results could improve if classes started later as studies have shown their body clock is about two hours "delayed" from older adults, he added.
Teenagers' "peak" is said to occur in the middle of the afternoon.
It is cruel to expect young adults to get up too early, he told the BBC.
"The older you get it seems the easier it is to get out of bed."
Professor Foster's conclusions go against the belief that pupils are most alert in the mornings.
"A later start time would benefit the students," he said.
However, he acknowledged it might not be practical to start the school day much later than at present.
"But what you may want to think about is rather than cramming the intellectually demanding subjects in first thing in the morning, shift them to early afternoon.
"And perhaps we should shift the exams to early in the afternoon."
Professor Simons said if school start times were switched, "academic performance improves, depression drops and feeling of well-being improves".
He added: "What's really very interesting is that as performance for teenagers improves throughout the day for adults, aged 40 or so, it actually gets worse."
Professor Foster presented his findings in a talk Society, the Brain and Education: Body Clocks and Sleep at the University of Wales Institute in Cardiff.