The tendency to sleep at a particular time each day defines an individual鈥檚 鈥渃hronotype鈥�. Although profoundly influenced by genetics and light exposure, age-related body changes play a key role.
Puberty heralds a notable shift as . This trend continues until 19.5 years in women and nearly 21 in men. Then it gradually reverses. By 55 we wake at around the time we did as young children, approximately two hours earlier than as adolescents. So a 7 am alarm call for a teenager is equivalent to a 5 am start for someone in their 50s.
The precise reasons are unclear but correlate with the neural and hormonal changes of puberty and then the decline in these hormones with age.
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This has consequences. A study from the University of Toronto compared cognitive performance mid-morning and mid-afternoon in teenagers and adults. Test scores in teenagers increased by 10 per cent from mid-morning to mid-afternoon, while in adults they declined by 7 per cent.
Timetable dilemma
These findings highlight an important dilemma. Teachers in their 50s will generally be at their best in the morning, but not so their teenage students, and it is the teachers who determine the timetables. The tacit assumption for over a century has been that students are most alert in the morning and this is when the most demanding subjects should be taught. This assumption is wrong for most teenagers.
An adolescent鈥檚 chronotype also leads to another problem 鈥� . Teenagers need approximately nine hours of sleep for full academic performance. Many get far less, going to bed late on a school night and getting only five hours of sleep before the alarm, or frustrated parents, drive them out of bed.
But the circadian system is not entirely to blame. also arises from more relaxed attitudes to bedtimes in recent years; ignorance regarding the importance of sleep across all ages; and the .
. Sleep promotes memory consolidation and problem-solving, while shortened sleep drives impulsivity, decreased empathy, self-harm, depression, and increased use of stimulants such as caffeine and nicotine and sedatives like alcohol.
Long-term lack of sleep has also been proposed as an important predisposing factor in 聽type 2 diabetes, obesity and hypertension 鈥� all now alarmingly common in adolescents.
Powerful arguments
In the US, such observations have led to . , for all its middle and high schools. The logic has been reinforced recently by the RAND research organisation, which estimates that a later school start time as a result of improved academic performance and reduced accident rates. Powerful arguments indeed.
So what about the UK? It is important to stress that the earliest start being proposed in the US is 8.30 am. In the UK most teenagers already start classes around 9 am. , such as 10 am? Possibly 鈥� but definitive large-scale studies comparing a 9 am and 10 am start have not yet been done.
In addition, the power of introducing sleep education into the school curriculum must also be considered. Teenagers do have a later biological chronotype, but this can be .
Educating teenagers about the importance of sleep, as we do about smoking, alcohol and sex, will at least allow them to make more informed choices.
Russell Foster is professor of circadian neuroscience and director of the Sleep and Circadian Neuroscience Institute, University of Oxford. He is co-author of (OUP).
