Forcing staff to start work before 10am is tantamount to torture and is making employees ill, exhausted and stressed, an Oxford University academic has claimed.

Before the age of 55, the circadian rhythms of adults are completely out of sync with normal 9-to-5 working hours, which poses a "serious threat" to performance, mood and mental health.

Dr Paul Kelley, of Oxford University, said there was a need for a huge societal change to move work and school starting times to fit with the natural body clock of humans.