pt en

know more

Most healthy adolescents experience a change in sleep behaviour, including longer periods of sleep and a tendency to delay bedtime. The implications of reduced sleep during this stage of development are a hot topic of research for cognitive neurosciences, and have already had repercussions; in October 2019, the state of California approved a pilot project to delay school start times.

The irregularity of adolescents’ sleep patterns is usually a cause of friction in many households. Staying up late, resisting waking up with the alarm clock and sleeping longer on weekends is the typical description of an adolescent’s routine. However, in recent years, sleep studies revealed that the biological clock makes an adjustment and also advances in adolescence. This modification is due to the biological transformations and psychological influences that occur at this stage of development.

Adolescents’ sleep practices that involve not getting the recommended full night’s sleep are common everywhere in the world1-4, and Portugal is no exception5. The national project Sono-Escolas analysed the sleeping habits of 400 adolescents aged between 13 and 18 years old , and identified substantial irregularities between the subjects’ bedtime and waking up hours, as well as their total sleep time, either during the week (with an average sleep time of 7 hours and 41 minutes, which is below the recommended time), and during the weekend (with an average sleep time of 9 hours and 46 minutes; that is, two more hours than during the week). In terms of self-perception, the Portuguese adolescents considered they were not getting enough sleep (60.8%). During the week, the most frequent causes were “noise in the room” and “thinking about problems” and, during the weekend, “watching television” and “ partying” until late at night6.

A maturation of the homeostatic and circadian processes that regulate sleep has been identified in adolescents’ healthy sleep patterns7. This means that, unlike children, adolescents tend to be more alert and stay alert, until later at night, which increases their resistance to sleep. Usually, this happens because, in adolescence, the production of melatonin (the sleep hormone that causes sleepiness) is secreted later in the evening. In the morning, adolescents still have high levels of melatonin and, as a consequence, they feel sleepy (some even feel sleepy in the middle of the day, regardless of their sleeping habits). An explanation for the delay in the circadian rhythm during puberty is the fact that adolescents have a longer intrinsic period (i.e., a longer internal day length)8. Some authors describe how the combination of these maturational changes leads adolescents to sleep later, while the demands of society, such as school start times, result in an insufficient sleep pattern due to lack of time for sleeping9. Therefore, it seems that the biological time of adolescents is out of sync with the early wake times required by society in general, and school in particular. This situation has been worrying sleep specialists, who believe that brain activity, while sleeping, can provide a unique window for the cortical maturation in adolescents10,11, as it is also during this resting period that the body produces essential proteins for the brain12.

The abusive use of electronic devices (e.g. smartphones and tablets) —  which is a reflection of natural bedtime procrastination —  is also harmful, as the exposure to the blue light emitted by these devices at night prevents the natural production of melatonin and awakens the brain circuit as if it was receiving warning signs13,14. Regardless of these practices, it is natural that a teenager does not easily become a “morning person”.

Adjusting social time

In recent years, several studies carried out in the UK and in the USA  demonstrated that teenagers who started their school day approximately one hour later had an improved attendance, academic performance (with a 4.5% increase in grades) and general health15,16. Furthermore, this change in the institutional organisation did not imply significant costs17.

The change in the development of rhythms during the transition to adolescence can thus provide information about individual differences in school performance as well as behavioural problems18,19. Studies with children aged between 11 and 14 years old, who were assessed both in the morning and in the afternoon, concluded that the children performed better in executive functioning measures when they were tested at their ideal (versus non-ideal) times of the day, even after controlling for their regular sleep duration and the sleep time of the previous night20. Most of the older children, for whom the level of excitement is higher at the end of the day, showed better executive functioning when tested in the afternoon, while the majority of the younger children, for whom excitement is higher at the beginning of the day, showed better executive functioning when tested in the morning.

In light of these data, it is inevitable to reflect on how this evidence is compatible with school start times with the vast majority of classes starting at 8 a.m. for all students. Following this logic, we are interfering with the biological time of many adolescents, who may be losing about two to three hours of sleep every night during the week. A recent meta-analysis studied the association between sleep duration and suicide ideation, concluding that the risk of suicide in adolescents decreased statistically (about 11%) for every extra hour of sleep21.

Scientific evidence on the damage caused by sleep deprivation at this stage of development has already led several schools in the USA to delay school start times and, recently, the state of California has also implemented a bill providing that all public schools (corresponding to basic and secondary education in Portugal), only start classes as of 8.30 a.m.22

The debate on adjusting school hours to those who are its primary recipients is not new, but the latest research supporting the benefits of late school start for teenagers has rekindled the discussion between scientists, health professionals, school communities and educational leaders. Although neurobiology may not explain everything, the fact that changes in sleep patterns at these ages are biologically determined and evidenced by scientific literature cannot be ignored. Schools should, somehow, take into account this convergence of biological, psychological and sociocultural influences.


1 Zhang, J., Paksarian, D., Lamers, F., Hickie, I. B., He, J., & Merikangas, K. R., «Sleep patterns and mental health correlates in US adolescents», The Journal of Pediatrics, 182, 2017, pp. 137–143.

2 Dorofaeff, T. F., & Denny, S., «Sleep and adolescence. Do New Zealand teenagers get enough?», Journal of Paediatrics and Child Health, 42(9), 2006, pp. 515–20.

3 Gibson, E. S., Powles, A. C., Thabane, L., et al., «“Sleepiness” is serious in adolescence: two surveys of 3235 Canadian students», BMC Public Health, 6, 2006, 116.

4 Yang, C. K., Kim, J. K., Patel, S. R., et al., «Age-related changes in sleep/wake patterns among Korean teenagers», Pediatrics, 115(1 Suppl), 2005, pp. 250–6.

5 Paiva, T., Gaspar, T., & Matos, M. G., «Sleep deprivation in adolescents: correlations with health complaints and health-related quality of life», Sleep Medicine, 16(4), 2015, pp. 521–527.

6 Pinto, T. R., Pinto, J. C., Rebelo-Pinto, H., Paiva, T., «O sono em adolescentes portugueses: Proposta de um modelo tridimensional», Análise Psicológica, 4 (XXXIV), 2016, pp. 339–352.

7 Tarokh, L., Raffray, T., Van Reen, E., Carskadon, M. A., «Physiology of normal sleep in adolescents», Adolescent Medicine, 21(3), 2010, pp. 401–17.

8 Carskadon, M. A., Vieira, C., & Acebo, C., «Association between puberty and delayed phase preference», Sleep, 16(3), 1993, pp. 258–62.

9 Carskadon, M., «Sleep in Adolescents: The Perfect Storm», Pediatric Clinics of North America, 2011, pp. 637–647.

10 Tarokh, L., Saletin, J. M., & Carskadon, M. A., «Sleep in adolescence: Physiology, cognition and mental health», Neuroscience & Biobehavioral Reviews, 70, 2016, pp. 182–188.

11 Crowley, S. J., Van Reen, E., LeBourgeois, M. K., Acebo, C., Tarokh, L., et al., «A longitudinal assessment of sleep timing, circadian phase, and phase angle of entrainment across human adolescence», PLoS ONE 9(11), 2014, e112199.

12 Cirelli, C., & Tononi, G., «Linking the need to sleep with synaptic function», Science, 366 (6462), 2019, pp. 189–190.

13 Hale, L., & Guan, S., «Screen time and sleep among school-aged children and adolescents: A systematic literature review», Sleep Medicine Reviews, 21, 2015, pp. 50–58.

14 Orben, A., & Przybylski, A. K., «The association between adolescent well-being and digital technology use», Nature Human Behaviour, 3, 2019, pp. 173–182.

15 Dunster, G. P., de la Iglesia, L., Ben-Hamo, M., Nave, C., Fleischer, J. G., Panda, S., & de la Iglesia, H. O., «Sleepmore in Seattle: Later school start times are associated with more sleep and better performance in high school students», Science Advances, 4(12), 2018, eaau6200.

16 Boergers, J., Gable, C. J., & Owens, J. A., «Later school start time is associated with improved sleep and daytime functioning in adolescents», Journal of Developmental & Behavioral Pediatrics, 35(1), 2014, pp. 11–17.

17 Kelley, P., Lockley, S. W., Foster, R. G., & Kelley, J., «Synchronizing education to adolescent biology: “let teens sleep, start school later”», Learning, Media and Technology, 40(2), 2014, pp. 210–226.

18 Wolfson, A. R., & Carskadon, M. A., «Understanding adolescent’s sleep patterns and school performance: a critical appraisal», Sleep Medicine Reviews, 7(6), 2003, pp. 491–506.

19 Goldstein, D., Hahn, C. S., Hasher, L., Wiprzycka, U. J., Zelazo, P. D., «Time of day, intellectual performance, and behavioral problems in morning versus evening type adolescents: Is there a synchrony effect?», Personality and Individual Differences 42(3), 2007, pp. 431–440.

20 Hahn, C., Cowell, J. M., Wiprzycka, U. J., Goldstein, D., Ralph, M., Hasher, L., & Zelazo, P. D., «Circadian rhythms in executive function during the transition to adolescence: the effect of synchrony between chronotype and time of day», Developmental Science, 15(3), 2012, pp. 408–416.

21 Chiu, H.-Y., Lee, H.-C., Chen, P.-Y., Lai, Y.-F., & Tu, Y.-K., «Associations between sleep duration and suicidality in adolescents: A systematic review and dose–response meta-analysis», Sleep Medicine Reviews, 42, 2018, pp. 119–126.



Subscribe to our Newsletter

Keep up with all the news