 As you're younger, you kind of go to bed at very different times of night, and as you get older, that starts to settle into a much more rigid pattern of going to bed slightly earlier and also getting up earlier. But also as you go through life, you're getting more and less sleep, so quite a lot in your earlier years and into your teenage years. It really dips, and it dips quite badly for middle-aged men particularly. They're the ones getting the least sleep out of anybody in this study, and then you start to get slightly more again after that.