According to them, about six-and-a-half hours sleep is OK, but people who regularly have more than eight hours a day could be doing themselves harm.
But too little isn't good for you either.
15 hours a night
Former Prime Minister Margaret Thatcher famously survived on four hours a night, but sleep experts say that's not enough for most people.
But Vanilla Sky actress Penelope Cruz says her beauty secret is that she gets up to 15 hours a night.
Hear Story
Try our sleep quiz
