File photo: 'Our findings suggest that adolescents’ chronotypes are on average too late for school start times before 8.30am.'

Teenagers are notorious for being impossible to drag from their beds in the morning.

But according to a study, they actually do need a lie-in of up to 90 minutes longer than their parents.

US scientists found teenagers have a later body clock, meaning they are night owls who go to bed late and wake up late.

The optimum bedtime for 17 and 18-year-olds is 12.30am, while they should wake up at 8.30am. This is in contrast with 60-year-olds, who typically go to bed at 11pm and rise at 7am.

However, from the age of 19, young adults begin to naturally get up earlier. It is not known why this is. Women also appear to wake earlier than men until the age of 40, when changes in their hormones – or lifestyle – may shift their sleeping patterns later.

The study, led by Harvard T.H. Chan School of Public Health, looked at data gathered from almost 54 000 people between 2003 and 2014 to determine their "chronotype" – or biological clock. 

Writing in the journal PLOS One, the authors said: "Our findings suggest that adolescents’ chronotypes are on average too late for school start times before 8.30am."