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Teenagers' body clocks really do run later. During adolescence the times people naturally fall asleep and wake up shift later, peaking around the age of 20, then drift earlier through adulthood. This personal timing, called chronotype, is shaped by both genes and age.

Roenneberg et al., 2004

HealthMind

The story behind the card

The age pattern comes from a 2004 survey of about 25,000 people in Central Europe led by the chronobiologist Till Roenneberg, which found that people are at their latest around 20; the researchers suggested this turning point could mark the biological end of adolescence. Chronotype is often measured with the Morningness-Eveningness Questionnaire, devised by James Horne and Olov Östberg in 1976. Genes matter too: a 2016 study of 128,266 people led by Samuel Jones found 16 genetic regions linked to being a morning person, some near known body-clock genes, and a 2019 study of nearly 700,000 people raised the count to 351. Light, school and work hours also shape sleep, which is why school-start-time research treats early timetables as a biological as well as a social issue.

Türkçe oku
Ergenlerin biyolojik saati gerçekten daha geç işler. Ergenlikte insanın kendiliğinden uykuya daldığı ve uyandığı saatler ileri kayar, 20 yaş civarında en geç noktaya ulaşır, yetişkinlikte yeniden erkene çekilir. Kişiye özgü bu zamanlamaya kronotip denir; hem genlerden hem yaştan etkilenir.
Roenneberg ve ekibi, 2004

Yaşla ilgili bu örüntü, kronobiyolog Till Roenneberg'in 2004'te Orta Avrupa'da yaklaşık 25 bin kişiyle yaptığı ankete dayanır. Buna göre uyku saatleri 20 yaş civarında en geç noktasına ulaşıyor; araştırmacılar bu dönüm noktasının ergenliğin biyolojik olarak bittiğini gösterebileceğini öne sürdü. Kronotip çoğu zaman, James Horne ve Olov Östberg'in 1976'da geliştirdiği Sabahçıl-Akşamcıl Anketi ile ölçülür. Genlerin de payı var: Samuel Jones önderliğinde 128.266 kişiyle yapılan 2016 tarihli araştırma, sabahçıl olmakla bağlantılı 16 gen bölgesi buldu; bunların bazıları bilinen iç saat genlerinin yakınındaydı. Yaklaşık 700 bin kişiyle yapılan 2019 tarihli bir çalışma bu sayıyı 351'e çıkardı. Işık, okul ve iş saatleri de uykuyu etkiler; okul başlama saatlerini inceleyen araştırmaların erken ders saatlerini yalnızca toplumsal değil, biyolojik bir sorun olarak da ele almasının nedeni budur.

Sources and further reading

Background reading: Chronotype
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The idea that the face can feed back into feelings, called the facial feedback hypothesis, goes back to Charles Darwin and William James. In a famous 1988 experiment, Fritz Strack and colleagues had people hold a pen in their teeth, which forces a smile, or between their lips, which prevents one, and those in the first group rated cartoons as funnier. In 2016, seventeen laboratories repeated that experiment exactly and found no effect. A 2019 review of 138 studies concluded that the effect is real but small and varies from study to study. The 2022 Many Smiles Collaboration, led by Nicholas Coles and designed jointly by supporters and sceptics of the idea, found that deliberate smiling could both strengthen and start feelings of happiness, while the pen method remained inconclusive.

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