Adolescent sleep has declined significantly over the past 20 years (Keyes, Maslowsky, Hamilton & Schulenberg 2015). Inadequate and poor-quality sleep appears to be associated with both reduced motivation (Zhao et al. 2019) and impaired cognitive abilities that are important for learning and academic performance (Hysing, Harvey, Linton, Askeland & Sivertsen 2016; Kuula et al. 2015).

Developmental hormonal changes shift the sleep-wake cycle towards eveningness during adolescence. Morningness starts to decline around 12 years of age, continuing until late adolescence and early adulthood (Roenneberg et al. 2007). Many adolescents have difficulty falling asleep at the desired time on school nights. A Finnish population-based study showed that the later adolescents went to bed, the lower their sleep quality and the greater their daytime sleepiness was. This in turn was reflected in lower school performance and motivation (Merikanto et al. 2013). The effects of sleep deprivation and poor sleep quality extend to all areas of life, including learning, motivation, and well-being. Moreover, inadequate and poor-quality sleep increases daytime sleepiness, which can be reflected in lower school performance and motivation.

The use of various screens in the evening is unfortunately common among young people, delaying their bedtime (Bartel, Gradisar & Williamson 2015). Digital media use affects young people through a variety of mechanisms, such as exposure to blue light (Crowley, Cain, Burns, Acebo & Carskadon 2015) and emotions which increase alertness (Scott & Woods 2019). Although circadian rhythms operate independently of environmental factors, artificial light can modify individual sleep-wake rhythms (Gooley 2008; Roenneberg, Daan & Merrow 2003). Daytime exposure to light is preferable, as exposure to light during the evening or night inhibits melatonin release in the evening, making it more difficult to fall asleep. Exposure to blue light before bedtime may also affect sleep architecture, for example by shortening REM (rapid eye movement) sleep (Higuchi et al. 2005), which is crucial for the development of the young brain and also affects the ability to learn new things (Li et al. 2017).

The use of digital media devices both during the day and at night has been associated with insufficient sleep in previous studies. It is therefore important to investigate how young people's circadian rhythms and sleep are associated with the use of digital devices and apps. This study targeted to investigate how sleep and circadian rhythms are related to digital technology use at school and during leisure time. Q1: How do sleep and fatigue during the school week differ across chronotypes? Q2: How is the amount of use of digital devices or apps during schooldays and leisure time associated with sleep and fatigue in adolescents?

The data consisted of population-based longitudinal data from three measurement points (gathered in 2021-2022) and an experience-based sample from a Finnish school during one school week. The DigiVOO longitudinal study followed adolescents in grades 7-9 (n = 6522). The number of respondents in the experience sample was n = 140. The one-week data collection for the experience sample was carried out in February 2022. Adolescents received mobile questionnaires after each lesson to assess their motivation and well-being. Circadian rhythms were assessed with a single question from the Morningness-Eveningness Questionnaire (Horne & Östberg 1976): “There are so-called morning-types and evening-types, which group do you belong to?”. A minority of respondents in the experience survey consider themselves to be definitely morning-types or more morning than evening-type compared to the other chronotypes. Day types were reported by around 20% of both the experience sample and the follow-up data. The amount of evening types was pronounced in both data sets. Almost half of the young people in the experience sample and just over a third of the young people in the follow-up data reported being more evening than morning types. Around one-fifth of the adolescents in the experience sample and just over a quarter of the young people in the follow-up sample considered themselves to be definitely evening-type. Respondents of the experience sample (n = 140) reported their bedtimes and wake-up times for one school week. These were used to calculate the average length of sleep over the follow-up week. Fatigue in school mornings and days was measured by the question "Are you tired in school mornings/school days?" Sleep quality was measured by the question "How did you sleep last night?" School-related stress was measured by the question "Is your sleep interrupted because of school issues?". The use of digital media was measured by asking at the end of each lesson for a total of one school week whether and how many digital apps or devices were used in the lesson. Adolescents were also asked how many hours in total they spend per day playing games, watching videos, series, or movies, searching for information or following news online, connecting with friends, using social media, and creating content on social media. As the group sizes were relatively small, differences in bedtimes and sleep duration between different chronotypes were examined using the Kruskall-Wallis H-test. Differences between chronotypes in sleep quality, fatigue, and school stress were examined using the chi-squared test.

The most important finding from the follow-up data on leisure time use of digital devices and apps was that adolescents who reported themselves as definitely evening types were at higher risk than other chronotypes for more extensive use of digital devices and apps in leisure time, especially for watching videos, series or movies, using social media and actively communicating on apps. In this study, evening-types spent most of their time on digital media use in the form of watching videos, series or movies, social media, or active communication in apps. The finding supports previous research findings. As evening types may be chronically out of sync with their circadian rhythm, they may be at higher risk for the effects of late-night digital media use, especially in terms of sleep quality. Adolescents have been reported to be particularly vulnerable to the negative effects of screen time on a good night's sleep (Quante et al. 2019). This study also confirms previous findings that sleep problems are common among evening youth (Merikanto et al. 2017; Roeser et al. 2012). It is important to raise awareness of the importance of different circadian rhythms and sleep for young people's well-being and learning. The shift in circadian rhythms towards eveningness is most pronounced in adolescence. Most young people are naturally evening-types, which makes it particularly difficult to fall asleep at the desired time to get enough sleep before the school day begins. For this reason, evening media use is concentrated in this group.

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