The shrill sound of your alarm clock rouses you from a deep sleep, forcing you out of bed, groggy and bleary-eyed. If this sounds like your typical morning, it might not just be late nights that are to blame.

According to sleep expert Dr. Paul Kelley, of Oxford University, circadian rhythms (governing your 24-hour body clock) vary over a lifetime, 'meaning there is a different "best time" to wake up and carry out routine activities, depending on your age'. So, when should we be doing what?