单选题：根据材料，回答题：Why Teenagers Really Do Need an Extra Hour in Bed?

根据材料，回答题：
Why Teenagers Really Do Need an Extra Hour in Bed?
A) "Making teens start school in the morning is 'cruel' ," brain doctor claims. So declared a British newspaper headline in 2007 after a talk I gave at an academic conference. One disbelieving reader responded: " This man sounds brain-dead. "
B) That was a typical reaction to work I was reporting at the time on teenage sleep patterns and their effect on performance at school. Six years on there is growing acceptance that the structure of the academic day needs to take account of adolescent sleep patterns. The latest school to adopt a later start time is the UCL Academy in London; others are considering following suit.
C) So what are the facts about teenage sleep, and how should society adjust to these needs? The biology of human sleep timing, like that of other mammals, changes as we age. This has been shown in many studies. As adolescence 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 adolescence. On average this is two hours earlier than adolescents. This means that for a teenager, a 7 a.m. alarm call is the equivalent of a 5 a.m. start for a person in their 50s.
D) Precisely why this is so is unclear but the shifts related with changes in hormones (荷尔蒙) at adolescence 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, cell phones and so on, all of which promote alertness and eat into time available for sleep.
E) The amoount 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.
F) Mary Carskadon at Brown University in Providence. Rhode Island, who is a pioneer in the area of adolescent sleep, has shown that teenagers need about 9 hours a night to maintain full alertness and academic perforruance. My own recent observations at a UK school in Liverpool suggested many were getting just 5 hours on a school night. Unsurprisingly. teachers reported students dozing in class.
G) Evidence that sleep is important is overwhelming. Elegant research has demonstrated its critical role in memory improvement and our ability to generate wise sohitions to complex problems. Sleep disruption may increase the level of the stress. Excited behaviors, lack of empathy, sense of humor and mood are similarly affected. All in all, a tired adolescent is a moody, insensitive, angry and stressed one. Perhaps less obviously, sleep loss is associated with metabolic (新陈代谢的) changes. Long-term lack of sleep might be an important factor for negative conditions such as diabetes (糖尿病), overweight and high blood pressure.
H) Adolescents are increasingly using stimulants to compensate for sleep loss, and caf, feinated (含咖啡咽的) and/or sugary drinks are the usual choice. So a caffeinated drink late in the day delays sleep at night. Tiredness also increases the likelihood of taking up smoking.
I) In the US, the observation that teenagers have biologically delayed sleep patterns compared to adults prompted several schools to put back the start of the school day. An analysis of the impact by Kyla Wahlstrom at the University of Minnesota found that academic performance was enhanced, as was attendance. Sleeping in class declined, as did self-reported depression. In the UK, Monkseaton High School near Newcastle instituted a 10 am start in 2009 and saw a progress in academic perfomance. J) However, a later start by itself is not enough. Society in general, and teenagers in particular, must start to take sleep seriously. Sleep is not a luxury but a ftmdamental biological need, enhancing creativity, productivity, mood and the ability to interact with others.
K) ff you are dependent upon an alarm clock, or parent, to get you out of bed ; if you take a long time to wake up; if you feel sleepy and impatient during the day; ff your behavior is overly impulsive, it means you are probably not getting enough sleep. Take control. Ensure the bedroom is a place that promotes sleep-dark and not too warm-don't text, use a computer or watch TV for at least half an hour before trying to sleep avoid avoid bright lights. Try not to nap during the day, and seek out natural light in the morning to adjust the body clock and sleep patterns to an earlier time. Avoid caffeinated drinks after lunch.
L) It is my strongly held View, based upon the evidence, that the efforts of dedicated (专注的，投入的) teachers and the money spent on school facilities will have a greater impact and education will be more rewarding when, collectively, teenagers, parents, teachers and school governors start to take sleep seriously. In the universal language of school reports: we must do better.

In the US and UK, several schools that have delayed the start of the school day witnessed a progress in academic performance.