Chronotype of teenagers, or why lessons in schools should start at 11 am
Chronotype is a feature that determines what time of activity the body prefers. People with the morning chronotype are commonly called “morning larks” and people with the evening chronotype – “night owls“. Like all our traits, it is partly genetically determined, and partly other factors – social, cultural and environmental. And like any other feature – it has a normal distribution. The majority of people are characterized by an intermediate chronotype, and there are relatively few people with an extremely evening and extremely morning chronotype.
Although the chronotype is an individual feature, it also depends on age. Young children are usually morning larks, but from the age of 11, the sleep phase gradually shifts towards the evening. The latest rhythm occurs in people around 20 years of age, then slowly begins to move back towards the morning.
Adolescents go to bed later with age, because they do not feel drowsy before, but they can not move the time of getting up. The duration of sleep they need to get enough sleep is also not reduced. The result is clear – sleep deprivation. According to studies, people during puberty need about 9-10 hours of sleep per day, but about half of them sleep less than 8 hours. People between 12 and 14 years from Monday to Friday sleep on average 8-9 hours, and then sleep off on weekends sleeping 9.5-10.5 hours. Older teenagers, aged 15-18 during school days sleep on average 7-8 hours, and on weekends 8.5-10. The average teenager must get up for school at 7, but on the weekend when he/she can sleep according to his/her rhythm – he/she gets up between 9 and 10.
Sleepless teenager is a global problem. Studies indicating sleep deprivation in students are from Europe, North America, Asia and Australia. In the survey of Polish high school students, 44% of girls and 30% of boys admitted that at least once a week, they suffer from excessive drowsiness, and 11% have this problem every day. Only 3.5% of girls and 11% of boys have never had problems with daytime sleepiness.
Effects of sleep deprivation in students
It is not surprising that lack of sleep hinders learning. That is why teenagers with the evening chronotype achieve worse school results than students with the intermediate and morning chronotype. Research indicates that students could achieve better results if they could live in harmony with their circadian rhythm. One study on this topic was carried out by Goldstein and colleagues. Researchers conducted tests on fluid intelligence (genetically determined) and crystallized intelligence (acquired knowledge) in students with morning and evening chronotypes, in the morning (8-10 am) and afternoon (1-3 pm). In the crystallized intelligence test no differences were found, but in the liquid intelligence test, yes. Among those with the morning chronotype, those who performed tasks in the morning performed better, while among those with the evening chronotype, those who completed the test in the afternoon did better. The difference between those who completed the test at their own time and those who were solving problems too early or too late was about 6 IQ points. It is worth noting that the study was conducted in the summer, when students do not show such a large sleep shortage as during the school year, so it can be expected that if the test was done during school days, the observed differences would be greater.
In addition to school results, it has also been shown that students with an evening chronotype have more behavioral problems, including concentration problems and aggressive behavior than students with a morning chronotype who can sleep longer.
Why do teenagers go to bed so late?
One has heard a lot in recent years about the negative effects of technology development on society. It is not difficult to find opinions saying that students do not sleep, because they play late on the computer, login on Facebook or watch TV. This may be partly true, but only partly…
Already in 1913, a common lack of sleep due to the necessity of getting up to school was noticed among students. It was also observed that in the USA, where lessons in schools began at 9 o’clock, students showed less sleep deficiency than in Europe (Germany and England), where the typical start time was 8. The authors of the study – Terman and Hocking, also noticed that with age, the percentage of students who are unable to wake up in the morning but who have to be awakened to arrive at 9 am to school increases. Among children aged 6-11 years it was about 20%, among students aged 12-14 years about 25%, among teenagers aged 15-17 years 30-40%, and among 18-year-olds almost 50% had to be awakened. The researchers admitted that this could be due to having to work until late hours, which was part of the duties of some older teenagers, but also considered the possibility of biological conditioning of such changes.
The thesis about the biological condition of the sleep phase shift is supported by the fact that girls shift their sleep phase about a year earlier than boys. In addition, girls have the most evening chronotype around 19.5 years old, while boys around 21 years old, which corresponds to the later puberty period in boys than girls.
Changes of chronotype in animals
It is worth knowing that the shift of the chronotype during puberty is not specific to humans, but also occurs in other animals. Similar changes in the chronotype were seen, for example, in rhesus monkeys, but also in lower animals. Studies on rodents – nocturnal rats and day-to-day degus have shown that individuals during puberty have the highest activity at the end of the species-typical activity period (end of night in rats and early evening in degus). The sleep phase shift was 2-4 hours. Removal of gonads before puberty meant that in adolescence, individuals did not show a typical chronotype change, suggesting that this change is the result of sex hormones.
It was also observed that in male rats the sleep phase shift was greater than in females. Similar results have been observed in humans – both boys and girls move during puberty toward the evening, but boys shift more. The degus change of the chronotype occurred only in males (although the studies are not compatible here, in others it occurred in males and females).
A shift of the chronotype during puberty has also been seen in mice, chubs (fish), giant toads and psammomys (rodent). It is possible that the change of chronotype in animals is associated with the fight for resources. Juveniles who have no chance to get food with adults, stronger individuals, are active at other times, when they do not have to compete with them.
How to solve the problem of sleepless teens?
Personally, I think it is necessary to move the start hours of lessons in schools. Studies show that for over 100 years, teenagers have been forced to wake up in the morning and have been fighting with lack of sleep in class every day, because for some reason the school has been adapted to the few teenage larks. On the other hand, owls, which constitute the majority in this age, are not taken into account. Of course, proper sleep hygiene also matters. It is worth avoiding caffeine and strong light in the late hours, remember about physical activity during the day and take care of a regular lifestyle. Following such rules may help you fall asleep a little earlier, but it will not make night owls a morning lark. The chronotype is largely biologically conditioned and we have no way of changing it freely.
That is why I think that lessons in junior high and high schools should start at 11 am. For some teenagers it will be too early, some will wake up much earlier and they will have to do something by themselves before lessons, but for most of them this time will be right. It will allow them to sleep as many hours as their body needs and start lessons when their body is ready.
But even a slight shift in the start of the lesson can bring positive results. Studies of school students in which it was decided to postpone the start time of classes by 30-60 minutes showed that the number of teenagers complaining about sleep deprivation, difficulty getting up in the morning and fatigue while doing homework decreased by 20-30%. The number of students feeling unhappy, depressed and irritated also decreased by 20%. Students began to get better grades and caused about a dozen percent less car accidents.
- Escribano, C. i in. (2011). Morningness/eveningness and school performance among Spanish adolescents: Further evidence. Learning and Individual Differences, 22, 409-413. (pdf at researchgate.net)
- Hastings Hagenauer, M., Lee, T. (2012). The neuroendocrine control of the circadian system: Adolescent chronotype. Frontiers in Neuroendocrinology, 33, 211-229.
- Goldstein, D. i in. (2007). Time of day, intellectual performance, and behavioral problems in Morning versus Evening type adolescents: Is there a synchrony effect? Personality and Individual Differences, 42, 431-440. (text at nih.gov)
- Gradisar, M., Gardner, G., Dohnt, H. (2011). Recent worldwide sleep patterns and problems during adolescence: A review and meta-analysis of age, region, and sleep. Sleep Medicine, 12, 110-118. (pdf at researchgate.net)
- Randler, C. (2008). Morningness-eveningness comparison in adolescents from different countries around the world. Chronobiology International, 25(6), 1017-1028.
- Terman, L., Hocking, A. (1913). The sleep of school children: Its distribution according to age and its relation to physical and mental efficiency. The Journal of Educational Psychology, 4(3), 138-147.
- Zawilska, J. i in. (2011). Chronotypy a deprywacja snu u młodzieży licealnej. Neuropsychiatria i Neuropsychologia 6(3-4), 159-165.
Author: Maja Kochanowska