"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."
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.
So what are the facts about teenage slumber, 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 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 a.m. alarm call is the equivalent of a 5 a.m. start for people 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.
Mary Carskadon at Brown University, who is a pioneer in the area of adolescent sleep, has shown that teenagers need about nine hours a night to maintain full alertness and academic performance. My own recent observations at a U.K. school in Liverpool suggested many were getting just five hours on a school night. Unsurprisingly, teachers reported students dozing in class.
Evidence that sleep is important is overwhelming. Elegant research has demonstrated its critical role in memory consolidation and our ability to generate innovative solutions to complex problems. Sleep disruption increases the level of the stress hormone cortisol. Impulsive behaviors, lack of empathy, sense of humor, and mood are similarly affected.
All in all, a tired adolescent is a grumpy, moody, insensitive, angry, and stressed one. Perhaps less obviously, sleep loss is associated with metabolic changes. Research has shown that blood-glucose regulation was greatly impaired in young men who slept only four hours on six consecutive nights, with their insulin levels comparable to the early stages of diabetes.