Young children need more sleep than adults and tend to wake up earlier. A typical ten-year-old needs about ten hours' sleep. (13) you reach adolescence, your body clock changes. (14) teenagers need about nine hours' sleep, but wake up later. As an adult, you'll need about eight to eight and a half hours' sleep a night. When we are much older, in our 70s and 80s, we're less (15) to sleep deeply and we usually need less sleep. Each of us has our own personal body clock, (16) makes us more alert in the morning or more alert in the evening. You might be a 'lark' who likes to go to bed early and get up early, or you might be an '(17)' who prefers to go to bed late and get up late. Or somewhere in between.