The participants of this study were drawn from “The Study of Testing the Physical Fitness and the Physical Activity of Estonian Schoolchildren and the Factors That Affect It”, which was conducted in the spring semester of 2021/2022. The questionnaire was completed by 1892 schoolchildren in the 6th, 8th and 10th grades. However, 11 partially completed questionnaires were excluded from the final analysis. This left a total of 1881 adolescents, aged 12 - 17 years, of whom 917 were boys (48.8%) and 964 were girls (51.2%).

In this study, the terms “boys” and “girls” are used to refer to schoolchildren aged 12 - 17 years. This terminology is chosen for brevity in tables, and it reflects the common usage in research related to adolescents. While the terms are concise, they are intended to include all students within this age range, recognizing the diverse experiences and developmental stages that may be present.

In the academic year 2021/2022, a total of 39,004 students studied in Estonian schools in the 6th, 8th and 10th grades ( Haridusandmete Portaal, n.d. ). Our sample represents 4.8% of this.

25 randomly selected schools across Estonia participated in the above-mentioned study of Estonian schoolchildren. Schools in a large city, including the capital, as well as small town and rural schools were included in the study. The study was carried out by the lecturers and students of the School of Natural Sciences and Health of Tallinn University, who received the necessary specialized training in advance to conduct this study.

Before the survey, the school management was asked for permission to conduct the study, and based on the positive decision, the study was introduced to the pupils, who were able to leave their informed consent with the class teacher.

All children who agreed to participate in the study and whose parents had given passive consent filled out the self-reported questionnaire in the computer classroom of the school in the presence of the researcher. The questionnaire was completed in the Google Forms environment.

“The Study of Testing the Physical Fitness and the Physical Activity of Estonian Schoolchildren and the Factors That Affect It” was approved by the ethics committee of Tallinn University (application no.: 6-5.1/34).

All students participating in the study were introduced to the purpose and activities of the study in the classroom teacher’s lesson. Each student independently decided whether to participate in the study and informed the class teacher of their decision.

Participation in the study was entirely voluntary for all individuals involved.

In addition to the schoolchildren, their parents were also informed about the study via email (twice) and asked for permission to include their children in the study. If a parent did not consent to their child’s participation in the study, they were asked to send an email to the researcher at the respective school.

The class teacher forwarded both the students’ informed consents and the parents’ purchases to the person responsible for the study.

Each student was assigned a unique code, guaranteeing their anonymity and confidentiality. The data collected during the research was used in an aggregated form and cannot be traced back to any specific individual.

The survey questionnaire for Estonian schoolchildren was compiled by the authors of this article.

The questions related to health assessment and more frequent health complaints were derived from the Estonian HBSC study ( Oja et al., 2019 ):

The samples of the physical activity questions were taken from a similar survey that was conducted in Estonian schools in the 2017/2018 academic year ( Kuu et al., 2018 ):

The questions about the screen and sleep time come from an Estonian Children Personality Behavior and Health Study (ECPBHS) ( Harro et al., 2009 ):

As background information, students were asked to indicate their grade, age, and gender.

The IBM SPSS Statistics program was used for data analysis. The following statistical methods were chosen to create a robust framework for analyzing relationships within the questionnaire data. Cross-tabulations with the Chi-square test enabled the exploration of associations between categorical variables, while Spearman’s rank correlation measured the strength and direction of associations between ordinal or non-normally distributed variables. This combination of methods facilitates drawing meaningful conclusions about the relationships among the diverse variables in the study. To ensure logical consistency, the response options for the questions on health assessment and the frequency of health complaints were reversed before conducting the correlation analysis, so that a better health assessment would have a higher numerical value, and more frequent complaints would also be represented by a higher value. Significance level set at p < 0.05.

Before conducting the correlation analysis, a factor analysis was performed on the most commonly reported health complaints to examine how the different characteristics were grouped. In the initial analysis, all eight characteristics from the more frequent health complaints were loaded into a single factor (KMO = 0.896, p < 0.001). The same has been found in previous studies ( Ravens-Sieberer et al., 2008 ), and based on Haugland et al. (2001) , the answer variants of health complaints were added together to find a general relationship, finding their own sum for each participant. (All eight of the more frequent health complaints were also strongly related, p < 0.001)

To specifically highlight the indicators related to mental health, a two-factor analysis was applied, following the approach of Lovis-Schmidt et al. (2022) , who differentiated between somatic complaints and psychological complaints ( Lovis-Schmidt et al., 2022 ). Based on the results of the rotated component matrix, headache, stomachache, backache, and drowsiness/dizziness were grouped together and treated as somatic complaints, with their average score considered in the correlation analysis. Meanwhile, feeling low, irritability/bad mood, nervousness, and difficulties falling asleep were grouped together as psychological complaints, with their mean score also included in the correlation analysis.

Students aged 12 - 17 assessed their health as good in more than half of cases and excellent in about a quarter of students ( Table 1 ), while boys rated their health better than girls (χ² = 26.007, p < 0.001).

The gender difference was statistically significant for all eight of the most common health complaints, while girls complain about various problems more often than boys (p < 0.001).

Girls complain of daily headaches almost three times more often than boys ( Table 2 ), and among boys, more than half of the participants (52.8%) report that they rarely or never have headaches (χ² = 126.386, p < 0.001).

Of all the surveyed health problems, adolescents complained least about stomach aches ( Table 2 ). Once a week or more, self-reported abdominal pain occurred in about a fifth (20.8%) of the participants, but it occurred almost twice as often among girls (χ² = 165.364, p < 0.001).

Regarding backache, the gender difference was not as great as for headache and stomachache ( Table 2 ), but girls still complained of back pain more often than boys (χ² = 45.565, p < 0.001).

Among psychological complaints, boys complained the least about feeling low ( Table 2 ). 55.7% of boys rarely or never perceived a feeling low, the same indicator among girls is more than twice lower, i.e. 24.6% (χ² = 241.125, p < 0.001).

Irritability was the most common psychological complaint ( Table 2 ). Only 24.1% of respondents rarely or never complain about irritability/bad mood, and only 14.6% of girls confirmed that they have not felt irritability/bad mood during the last six months (χ² = 185.595, p < 0.001).

Of all assessed health complaints, including psychological complaints ( Table 2 ), nervousness was the most common among girls every day, where 27.1% of respondents chose the answer option “almost every day” (χ² = 195.862, p < 0.001).

Of all assessed health complaints, including psychological complaints ( Table 2 ), difficulties falling asleep was the most common among boys every day, where 11.5% of respondents chose the answer option “almost every day” (χ² = 83.380, p < 0.001).

Approximately one-third of adolescents (34.5%) reported experiencing drowsiness/dizziness at least once a week or more ( Table 2 ). Also in this indicator, the frequency of girls’ complaints is higher compared to boys (χ² = 118.103, p < 0.001).

Adolescents aged 12 - 17 did exercises mainly 3 - 5 times a week ( Table 3 ). The answer option “3 times a week” turned out to be the most popular choice, while the frequency of exercise for boys was higher compared to girls (χ² = 38.901, p < 0.001).

Also, among boys, the intensity of exercise was higher than among girls (χ² = 37.909, p < 0.001), but representatives of both sexes do exercise in such a way that it makes them sweat and breathe mostly 2 - 3 hours a week ( Table 4 ).

They were also physically active time outdoors, mainly 2 - 3 hours a week ( Table 5 ). For this indicator of physical activity, the boys’ results also exceeded those of the girls (χ² = 21.582, p < 0.001).

53.6% of boys and 62.5% of girls spent more than 4 hours a day on the screen ( Table 6 ; χ² = 18.736, p < 0.001).

Approximately one-third of girls aged 12-17 and one-fifth of boys slept less than 7 hours a day ( Table 7 ; χ² = 40.278, p < 0.001).

Self-assessment of health was moderately and inversely related to all indicators characterizing health complaints (p < 0.001), including the total number of health

complaints, psychological complaints, and somatic complaints ( Table 8 ). Similarly, self-assessment of health was positively related with all indicators of physical activity (p < 0.001). There were medium-strength associations with exercise frequency and hours of intensive exercise, while the association with time spent being physically active outdoors was weaker.

Both the sum of health complaints and psychological and somatic complaints were weakly and inversely related to all indicators of physical activity (p ≤ 0.001; Table 8 ).

Self-assessment of health was weakly and inversely related to screen time and weakly but positively related to sleep duration (p < 0.001). At the same time, all health complaint indicators were weakly and positively related to screen time and weakly and inversely related to sleep duration (p < 0.001; Table 8 ).

In addition, it can be pointed out that all indicators of physical activity were weakly and inversely related to screen time and weakly and positively related to sleep duration (p ≤ 0.01 - < 0.001). Screen time and sleep duration were also weakly and inversely related (p < 0.001; Table 8 ).

Age has a moderating effect on health-related indicators, screen time and sleep duration (p < 0.001; Table 8 ). Age showed a weak inverse relationship with health assessment and sleep duration, and weak positive relationships with all indicators of health complaints and screen time (p < 0.001).

In this study, 83.8% of boys and 76.5% of girls rated their health as good or

a. Spearman’s rho.

excellent. These results are consistent with findings from the Estonian and German HBSC surveys ( Oja et al., 2023 ; Reiß et al., 2024 ), firstly, boys tend to rate their health higher than girls, and as age increases, health assessment indicators decline. At the same time, there is a significant difference in the percentages of the “excellent” health rating. According to the Estonian HBSC survey reports that among students aged 11 - 15, 41% of boys and 29% of girls rate their health as excellent ( Oja et al., 2023 ). However, in this study, only 28% of boys and 20% of girls aged 12 - 17 rated their health as excellent.

The findings on health complaints in this study are also consistent with the results of the HBSC study conducted in Estonia and Germany ( Oja et al., 2023 ; Reiß et al., 2024 ), where girls had more complaints than boys, and the number of complaints increased with age. Similarly to Oja et al. ( Oja et al., 2023 ) and Bianco et al. (2019) , our study also showed that the most frequent health complaints among adolescents were irritability/bad mood and nervousness. At least once a week or more, 51.8% of respondents complained of nervousness and irritability/bad mood. Among somatic complaints, headache was the most common complaint - 35.7% of 12 - 17-year-old schoolchildren had a headache once a week or more. Headache was also the most frequent physical health complaint among Norwegian adolescents ( Nilsen et al., 2023 ). It is well established that health issues can disrupt concentration and learning abilities. Mental health, in particular, has a significant impact on adolescents’ academic success ( Agnafors et al., 2021 ).

The inverse relationship between self-assessment of health and health complaints further confirms the reliability of the self-reported questionnaire. Therefore, it can be concluded that if school students perceive more health problems, their health assessments tend to be lower. According to Corell et al. (2022) , the main factor causing health complaints is stress, which is associated with psychological complaints more than with somatic ones. But at the same time, Lovis-Schmidt et al. (2022) highlight in their study the fact that negative emotions predict the presence of health complaints more than stress and health behavior. On the other hand, participating in organized sports has been shown to enhance psychological well-being and help prevent mental health issues ( Tahira, 2022 ). This was also confirmed by our correlation analysis, which shows that the frequency of exercise and hours of intensive exercise are more positively related to health assessment than physically active time outdoors. Additionally, the correlation coefficients for the inverse relationships between the total number of health complaints, psychological complaints, and somatic complaints are higher for exercise frequency and hours of intensive exercise compared to time spent being physically active outdoors. Furthermore, psychological complaints did not differ from somatic complaints in terms of the strength of their connection. Although, several authors point out the beneficial effect of intensive exercise on more common mental problems such as anxiety and depression ( Neill et al., 2020 ; Panza et al., 2020 ; Tahira, 2022 ). However, Dahlstrand et al. state that any physical activity, regardless of intensity, reduces stress and stress-related mental health problems ( Dahlstrand et al., 2021 ). And according to Cheon (2021) , exercise frequency seems to be related to subjective happiness, which in turn contributes to mental well-being.

In addition to various indicators of physical activity, self-assessment of health and more frequent health complaints were weakly but still statistically significantly related to screen time and sleep duration. Moreover, in the case of the above-mentioned relationships, from the point of view of health, longer sleep duration is more important than shorter screen time. And in both cases, age was shown to have a noticeable effect on the results of this study. This means that as age increases, screen time tends to rise while sleep duration decreases, both of which contribute to more frequent health complaints. Conversely, reducing screen time and increasing sleep duration can lead to fewer health complaints. These results also coincide with previous studies, which revealed that if adolescents have less screen time, they also have fewer health complaints ( Hrafnkelsdottir et al., 2018 ; Khan et al., 2022 ; Marques et al., 2019 ). The inverse relationship between sleep duration and health complaints in this study also coincides with previous studies, where shorter sleep duration was associated with a greater number of health complaints ( Paiva et al., 2015 ; Segura-Jiménez et al., 2015 ). Bedtime also plays a significant role; the later adolescents go to sleep, the more health problems they experience ( Dutil et al., 2022 ; Gariépy et al., 2019 ). In addition to health complaints, insufficient sleep was also associated with emotional and behavioral problems ( Kosticova et al., 2020 ). The average required sleep duration for teenagers is 9 h ( Matricciani et al., 2013 ). According to our research, only 13.3% of participants fell into this category. Among boys, the situation was slightly better than among girls, 15.8% and 10.9% of whom sleep 9 - 10 hours a day, respectively. When we also consider whether screen time aligns with the recommendations (WHO suggests no more than 2 hours per day for teenagers), the reality becomes even more concerning. Only 5.1% of students aged 12 - 17 had less than two hours of screen time.

It appears that if adolescents participated in more exercise or spent more time being active outdoors, their screen time would likely decrease, their sleep duration would improve, the frequency of health complaints would diminish, and their overall health assessment would improve. This is exactly what the multifaceted correlative relationships of this study exposed. There is significant room for improvement in this age group, as only 8.4% of study participants engage in daily exercise, and just 12.6% of 12 - 17-year-old students meet the WHO recommendation of 7 hours of moderate and vigorous intensity activities per week ( WHO, 2024 ). In this study, boys performed better than girls on both indicators.

Previous research also supports our statement. Xu et al. (2019) confirm that adolescents whose screen time and physical activity meet WHO recommendations had better sleep quality and quantity. Sufficient sleep and sufficient physical activity in turn had a positive effect on mental health ( Wilhite et al., 2023 ). And if all three indicators, i.e. physical activity, sleep duration and screen time, meet the recommendations, the incidence of depression symptoms among adolescents also tends to decrease ( Sampasa-Kanyinga et al., 2020 ). Therefore, the entire combination of physical activity, screen time, and sleep duration plays a crucial role in adolescents’ self-reported health assessments and the frequency of their health complaints. This impact is equally significant for both psychological and somatic health complaints. It is also noteworthy that the most important physical activity indicators are exercise frequency and the duration of intensive exercise, while time spent being physically active outdoors is less influential.

Sample size: With 1881 participants, the study represents a relatively large sample, enhancing the statistical reliability and generalizability of the findings to a broader population of Estonian adolescents.

Age range: The study includes adolescents aged 12 to 17, encompassing a crucial developmental period during which health behaviors are established.

Scope: The research conducted is thematically diverse, examining several key variables known to impact adolescent health: self-assessment of health, health complaints, physical activity, screen time and sleep duration. Such a comprehensive approach provides a thorough understanding of adolescent health.

Focus on health complaints: By focusing on the most common health complaints, the study addressed an important and practical aspect of adolescent well-being which can have both immediate and long-term effects.

Practical value for the design of healthcare strategies: The results of the study provide valuable information for public health interventions targeting the health behaviors of adolescents in Estonia or similar population groups.

Self-reported data: Using self-reported data to generalize about health, physical activity, screen time, and sleep duration can lead to misinterpretation and individual-based bias. Adolescents may overestimate positive behaviors (e.g. physical activity) or underestimate them (e.g. screen time).

Lack of objective measures: Lack of objective measures (such as the use of wearable devices to monitor physical activity and sleep) may limit the accuracy and reliability of the data. In other words, a subjective assessment may not be generalizable.

Cross-sectional study:

This study can only detect correlations between variables, not causation. For example, in the case of the relationship between screen time and health complaints, we cannot determine whether increased screen time causes more health complaints or vice versa.

The data provide an overview of adolescent health at a limited point in time, which may not reflect changes or trends over a longer period of time.

Cultural and contextual constraints:

Limited generalizability: Although the study is valuable for understanding the health behavior of Estonian adolescents, the findings may not be generalizable to adolescents living in other countries or cultural contexts without additional validation studies.

Contextual factors: The study does not take into account broader social, economic and environmental factors that could influence the health behaviors and outcomes of the adolescents studied.

Developmental differences: Adolescents between the ages of 12 and 17 are at different stages of physical, cognitive, and emotional development. The study may not account for these differences, which may affect the relationship between the variables studied.

Uncontrolled variables: The relationships between health, physical activity, screen time, and sleep duration are also influenced by other important factors (eg, diet, socioeconomic status, mental health) that the study does not control for, which may affect the results.