The biology of human sleep timing, like that of other mammals, changes as we age. This has been shown in many studies. As puberty begins, bedtimes and waking times get later. This trend continues until 19.5 years in women and 21 in men. Then it reverses. At 55 we wake at about the time we woke prior to puberty. On average this is two hours earlier than adolescents. This means that for a teenager, a 7 am alarm call is the equivalent of a 5 am start for a person in their 50s.
Precisely why this is so is unclear but the shifts correlate with hormonal changes at puberty and the decline in those hormones as we age.
However, biology is only part of the problem. Additional factors include a more relaxed attitude to bedtimes by parents, a general disregard for the importance of sleep, and access to TVs, DVDs, PCs, gaming devices, cellphones and so on, all of which promote alertness and eat into time available for sleep.
The amount of sleep teenagers get varies between countries, geographic region and social class, but all studies show they are going to bed later and not getting as much sleep as they need because of early school starts.
Sleep is not a luxury or an indulgence but a fundamental biological need, enhancing creativity, productivity, mood and the ability to interact with others. Learn more here.