Data collection was conducted through an electronic survey (Google Forms) during the COVID-19 pandemic in both countries. All questionnaire items were mandatory. For all forms of data collection, all information and the same instructions were provided to participants. The questionnaire was shared in Czech Republic in the Czech language and in Slovak Republic in the Slovak language. The selected scales were translated from English to Slovak. The verification of the translation was carried out by assessing the English and Slovak versions by three experts. After incorporating the comments, the scales were sent to a group of students (n = 20) who were asked to evaluate the comprehensibility of the items. After incorporating the students' comments, the questionnaire was translated from Slovak into Czech. The translation and clarity were assessed by three experts (assessing the English, Slovak, and Czech versions) and the comments were subsequently incorporated. The Slovak version was chosen for translation into Czech on the basis of the great similarity between the Slovak and Czech languages. The national language versions of the questionnaires are provided in Supplementary Tables 1–4.

The data collection took place in two phases. The first phase was implemented in both countries from March to May 2020, while the second phase was implemented after the summer vacation, i.e., from October to December 2020. It should be noted that ~80% of all data were collected in both countries during the first phase. In the first phase, college and university authorities (deans, vice deans, study officers) as well as student councils were contacted by emails and asked to provide a questionnaire to students with a request to complete it. In this phase, the questionnaire was also shared on social media not only in an organic way (free sharing in student groups and on fan pages) but also in a paid way (paid advertising). In the second phase, emails were sent to teachers (addresses were obtained from publicly available contact databases) asking them to share the questionnaire with students of specific study fields at individual colleges and universities. The purpose was to complete the planned sample structure. In the data collection process, the effort was to obtain a research sample as representative as possible with respect to the population under study. Sample characteristics were fitted on the basis of two main criteria. Ensuring an appropriate representation of colleges and universities was a primary concern, and thus 80% of all colleges and universities in both countries were included in the research. At the same time, fields of study were controlled in order to obtain a reasonable proportion of study fields with a minimum of 30 observations.

In the data cleaning process, the following numbers of statistical units were excluded from the sample: 179 (5.3%)—based on a control item in the questionnaire to filter out “automated” responses (agreeing that one million has six zeros, while a numerical value of 1,000,000 was also given); 27 (0.8%)—based on the identification of a system error in the recording of responses; 87 (2.6%)—based on students' foreign nationality (the research was focused on domestic students only). The final research sample consisted of 3,099 participants (Czech Republic = 1,422; Slovak Republic = 1,677). Some identifying characteristics were incorrectly reported by the participants (e.g., 1,000 as the year of birth); therefore, these responses were deleted and treated as missing data in the analyses. Table 1 presents the sample structure separately for Czech Republic (CZ) and Slovak Republic (SK).

All aspects of the research were conducted in adherence to the Declaration of Helsinki. The study was approved by the General University Hospital in Prague as individual research (Ref. 915/20 S–IV). Informed consent to participate in the research was provided by all study participants, who received adequate information about the study objectives, required data, and expected benefits. The participants were informed that their responses would not be judged and their confidentiality would be respected. They were also informed that all participation was confidential, anonymous, voluntary, and harmless. The participants did not receive any financial reward. Also, all study participants completed the questionnaire consisting of demographic data, items assessing Internet addiction, items assessing anxiety symptoms, items assessing depressive symptoms, and items assessing perceived stress.

The analyses included four measures of mental health (anxiety symptoms, depressive symptoms, stress, Internet addiction) based on existing scales presented in previous studies. The study conducted by Kroenke et al. (66) provided measures of anxiety and depressive symptoms. These were the Generalized Anxiety Disorder (GAD-7) as a 7-item scale, and the Patient Health Questionnaire for depressive symptoms (PHQ-9) as a 9-item scale. Both scales offered the following possible responses: (0) not at all, (1) several days, (2) more than half the days, and (3) nearly every day. The total GAD-7 score was the sum of the coded responses and its ranges indicated: no anxiety symptoms (0–4), mild anxiety symptoms (5–9), moderate anxiety symptoms (10–14), and severe anxiety symptoms (15 and more). The severity ranges of the total PHQ-9 score were as follows: no depressive symptoms (0–4), mild depressive symptoms (5–9), moderate depressive symptoms (10–14), moderately severe depressive symptoms (15–19), and severe depressive symptoms (20 and more).

Another measure was the Perceived Stress Scale (PSS) developed by Cohen et al. (67). This scale consisted of 10 items and offered the following responses: (0) never, (1) almost never, (2) sometimes, (3) fairly often, and (4) very often. The total PSS score was the sum of the coded responses and its ranges indicated low stress (0–13), moderate stress (14–26), and high stress (27–40).

The last fourth measure was the Internet Addiction Test (IAT) based on an existing scale developed by Young (68). This 20-item scale offered possible responses using a Likert scale: (0) not applicable to your life, (1) rarely, (2) occasionally, (3) frequently, (4) often, and (5) always. The total IAT score could take on a value in ranges indicating no addiction (a normal level of Internet usage) (0–30), mild addiction (31–49), moderate addiction (50–79), and severe addiction (80 and more). Based on the above-mentioned, the higher the total score, the more severe the mental health problem.

The level of reliability (Cronbach's alpha) was acceptable in all of the analyzed cases (i.e., higher than 0.7). For GAD-7, it was 0.878 in the Czech sample and 0.865 in the Slovak sample. For PHQ-9, it was 0.867 in the Czech sample and 0.861 in the Slovak sample. For PSS, it was 0.757 in the Czech sample and 0.708 in the Slovak sample. For IAT, it was 0.888 in the Czech sample and 0.899 in the Slovak sample.

The statistical analysis of the data consisted of several analytical procedures. First, frequency and descriptive analyses were used to provide a first look at the data using statistical measures such as mean, standard deviation, 25th percentile, median, 75th percentile, minimum, and maximum. Non-parametric tests of differences were also applied when assessing the descriptive dimension of the data. The Mann-Whitney U-test was used for differences between two categories and the Kruskal-Wallis H-test was used for differences between three or more categories. In subsequent analytical steps, the quantile regression analysis (with the 25th, 50th, and 75th percentiles) was used to evaluate the associations between Internet addiction (IAT) and mental health problems such as anxiety symptoms (GAD-7), depressive symptoms (PHQ-9), and stress (PSS). In addition, the binomial logistic regression analysis was chosen to assess the relationships between selected study-related characteristics and Internet addiction, anxiety symptoms, depressive symptoms, and stress in higher education students. In this case, the IAT, GAD-7, PHQ-9, and PSS scores were adjusted to a binomial form, with 0 representing the lowest range and 1 representing the other higher ranges of the total scores.

Czech and Slovak students showed significant differences in scores for anxiety symptoms, depressive symptoms, and stress. In this respect, Czech students reported higher mean scores indicating their more frequent mental health problems. This means that, despite shared social priorities and a common history in which these countries formed a single entity, each country behaves as a unique living organism and is characterized by its own social specificities. The importance of cross-cultural differences when examining the mental health of students from more than one country was also highlighted by Ochnik et al. (69). Additionally, this study revealed differences between most of the study-specific categories. All these results could be explained by the different measures implemented in the countries during the pandemic.

The overall prevalence of Internet addiction (IAT > 30) was 30.7% among Czech and 33.2% among Slovak students during the first wave of the COVID-19 pandemic. The most serious levels of Internet addiction requiring increased attention were identified in 3.5% (moderate: 3.4%, severe: .1%) and 6.2% (moderate: 6%, severe: .2%) of Czech and Slovak students, respectively. Zhu et al. (33) found a similar prevalence and revealed that 28.4% of Chinese students developed Internet addiction, which correlated with their academic burnout. In view of these results, the cut-off point for IAT was ≥ 50. Using the same cut-off point, a similar prevalence (5.8%) was found in students from Thailand (70), while a higher prevalence was observed in American students (16.8%) (31), Spanish students (12.4%) (71), Iraqi students (23%) (32), Bangladeshi students (32.6%) (72), Chinese students (28.4%) (73), Indian students (10.1%) (47), as well as Brazil students (20%) (74). Interestingly, Khazaie et al. (75) found up to 68.8% prevalence of Internet addiction among Iranian medical students. The mean IAT scores indicated a normal level of Internet usage in both countries examined in this study. In comparison, similar scores were reported by students from Turkey (76), while higher scores indicating a mild Internet addiction were found in students from Iran (41), Japan (77), and Australia (78). Overall, it was possible to conclude that Czech and Slovak college students reported less Internet addiction than students from other countries. A possible explanation is cultural differences, which were highlighted in the study by Lozano-Blasco et al. (26).

In Czech Republic, 40.3% of students suffered from anxiety symptoms (GAD-7 ≥ 5), of whom 26.2% met the criteria for mild anxiety symptoms, 9.2% for moderate anxiety symptoms, and 4.9% for severe anxiety symptoms. For Slovak Republic, anxiety symptoms occurred in 34.6% of students, with 23, 8.1, and 3.5% of students reporting mild, moderate, and severe anxiety symptoms, respectively. Using the standard cut-off point of GAD-7 ≥ 10, it was possible to identify 14.1% of Czech and 11.6% of Slovak students with anxiety symptoms requiring increased attention. Interestingly, Hajduk et al. (57) used the same cut-off point and found a considerable higher prevalence of anxiety symptoms among Slovak higher education students (43.3%). This can be explained by the fact that the research in this study was conducted during the first wave, when the situation in the country was not so critical. On the contrary, Hajduk et al. (57) conducted their research at the end of 2020, when the situation was much more serious and this may have been reflected in the mental health outcomes. A higher prevalence was also found in Saudi medical students (32.4%) (79), Pakistani medical students (30.3%) (80), Portuguese students (23.9%) (81), Polish students (38.4%) (82), Turkish students (44.5%) (83), and Brazilian students (53.8%) (84). The mean GAD-7 score was 4.71 for Czech students and 4.15 for Slovak students. Czech students reported a similar mean score also in the study conducted by Ochnik et al. (69) during the first wave of the COVID-19 pandemic. Compared to the study examining nine countries (38), higher mean scores were reported by students from Poland (9.2), Slovenia (7.37), Russia (7.48), Turkey (10.41), Israel (7.92), Colombia (8.45), and Ukraine (6.15), while only students from Germany reported a lower mean score (2.92). The mean GAD-7 score of Czech students was consistent with this study. A slightly higher mean score was found in China (5.38) (85). It is clear that students from Czech Republic and Slovak Republic suffered from anxiety symptoms during the COVID-19 pandemic to a lesser extent than students from other countries.

During the COVID-19 pandemic, depressive symptoms were prevalent in 52% of Czech students and 47.7% of Slovak students. More specifically, mild depressive symptoms occurred in the same proportion of Czech and Slovak students, i.e., in 28.6%. Moderate depressive symptoms were reported by 13.6% of Czech and 11% of Slovak students, while moderately severe depressive symptoms were reported by 6.4% of Czech and 5.4% of Slovak students. The most severe depressive symptoms were identified in a not negligible proportion of students, i.e., in 3.4% of Czech students and 2.7% of Slovak students. In terms of the cut-off point of PHQ-9 ≥ 10, 23.4% of Czech students and 19.1% of Slovak students had depressive symptoms, which needs to be addressed. As seen in anxiety symptoms, this study revealed a lower prevalence of depressive symptoms among Slovak students than the study by Hajduk et al. (57), who conducted their research at the peak of the second wave of the pandemic, when the situation was more dramatic than in the first wave. Slovak students also reported more depression than anxiety, which is in line with the results revealed by Hajduk et al. (57). In comparison, a similar prevalence was observed among Indian doctoral students (26.7%) (86), Chinese students (15.8%) (87), and Korean students (14%) (88), while a higher prevalence was observed among students from Germany (37%) (89), Bangladesh (48.8%) (90), and Malaysia (33.8%) (91). The mean score was 6.34 for Czech students and 5.77 for Slovak students, indicating mild depressive symptoms. Based on the mean score, mild depressive symptoms were also reported by German (6.77) and Chinese (6.99) students (85). A higher mean score reported Bangladeshi students (9.5) (90), Polish students (11.3) (92), and Greek students (9.36) (93).

In Czech Republic, 74.1 and 12.9% of students reported moderate and high stress, respectively. In Slovak Republic, 79.4% of students reported moderate and 9.1% high stress. Using the cut-off point of PSS ≥ 27, a higher prevalence of stress showed students from India (31.5%) (94) and Saudi Arabia (19.6%) (79). On the other hand, a similar prevalence was found in Sri Lanka (11.8%) (95) and Polish students (43%) (96). The mean PSS score was 19.82 in Czech Republic and 19.28 in Slovak Republic. The perception of stress by Slovak students in this study was in line with the study by Rutkowska et al. (61). A slightly higher mean score was reported by French students (21.9) (97). When comparing the results of this study with those of the study on the mental health of students from nine countries (38), similar mean scores were found in Slovenia (19.83), Ukraine (19.93), Israel (21.51), and Colombia (21.37), while slightly higher mean scores were found in Turkey (22.71), Poland (22.69), Germany (22.54), and Russia (21.98). Czech students' perception of stress was consistent with this study (18.16).

The quantile regression analysis used in this study revealed the significant positive associations of Internet addiction with anxiety symptoms, depressive symptoms, and stress. These findings indicated that higher scores of IAT were associated with higher scores of GAD-7, PHQ-9, and PSS, regardless of the rate of the mental health problems. The stated hypotheses H1a, H1b, and H1c were supported. On this basis, it was possible to agree that Internet addiction is associated with other mental health problems in young people, pointing to a threatening element in their lives. Similar findings can be found in previous studies from other countries (28, 29, 31, 32, 98). In this context, problematic Internet use may be a predictor in screening high-risk students for mental health problems (30). Jiang et al. (27) also revealed that internet addiction was directly related to college students' depression and indirectly predicted students' depression via the mediator of social support and sleep quality. On the other hand, under the mediation of fear of missing out, young people with anxiety are more likely to develop Internet gaming disorder, while young people with depression or stress may be susceptible to other types of Internet use disorders (99). In this context, Yang et al. (100) confirmed that baseline Internet addiction had a significant net-predictive effect on follow-up depression, but it was also true that baseline depression had a significant net-predictive effect on follow-up Internet addiction. Thus, the issue can also be considered from the opposite perspective, in which mental health problems such as anxiety, depression, and stress are associated with Internet addiction in students (47, 72–74, 77).

The co-occurrence of Internet addiction and anxiety symptoms, depressive symptoms, and stress in students suggests that Internet addiction is positively associated with other mental health problems (41). The results could be explained by the ongoing pandemic and by the fact that excessive use of the Internet impairs psychological wellbeing, which can be a driver of psychological distress (27–31). Particularly in the context of the COVID-19 pandemic, the Internet has been used as an effective tool to provide distance education. In this way, the pandemic may have raised concerns about the mental health of young adults and contributed to the development of problematic Internet use by students, which may have further undesirable consequences (101). The Internet can provide a short-term reward during difficult times, but it can also lead to other behavioral addictions (102), with adolescence and young adulthood being the most vulnerable period of life (103). It is important to note that although digital technologies have dramatically changed the environment in which college students connect with each other, gain knowledge, and have fun, they also appear to have some detrimental effects on mental health (24). In addition, these problems adversely affect both academic performance and quality of life and lead to many other social and health consequences (48–53, 55, 56, 104–106). For these reasons, digital technologies should be used with caution, the higher education environment should adapt to this situation (107), and students at risk of Internet addiction should also be monitored for other psychological problems.

This study revealed that first- and second-degree students from Czech Republic and Slovak Republic were less likely to suffer from anxiety symptoms than third-degree students. In addition, especially Czech second-degree students were less prone to depressive symptoms compared to third-degree students. This could be due to the fact that the third degree of study is considered to be very demanding and challenging for students. Finally, it was found with caution that mainly Czech students of the combined study were more likely to be addicted to the Internet compared to third-degree students. Overall, it can be concluded that the degree of study is a factor that needs increased attention, especially in terms of anxiety. In view of the results, there is a need to monitor third-degree students, as emphasized by Kowalczyk et al. (37). Other studies have also shown that the study degree is an important factor in examining mental health problems among students (38, 39).

This study also contributes to the knowledge that full-time study can be considered a risk factor for students' mental health, as indicated in the studies conducted by Aristovnik et al. (39) or Stallman (40). This fact was observed among both Czech and Slovak students. More specifically, Czech full-time students were more likely to have Internet addiction, anxiety symptoms, depressive symptoms, and stress compared to part-time students, while Slovak full-time students were more likely to have Internet addiction and depressive symptoms than part-time students. A possible explanation may be that full-time students were more vulnerable in terms of the COVID-19 pandemic (108). Also, full-time students are generally under more pressure in the higher education environment. However, the opposite result was found by Esmaeelzadeh et al. (109), in which part-time students were considered a risk group. In any case, the findings of this study can be followed up with further insights into the form of study and mental health in the college environment.

With a focus on the field of study, most of the results indicated that students of Informatics, Mathematics, and ICT were more prone to mental health problems than students of other fields. These findings are indicative of the fact that the field of study should not be underestimated and overlooked when examining students' psychological problems. This has also been demonstrated in other studies, the results of which differ from those of this study (3, 41). For instance, in the study by Lipson et al. (42), students of Humanities and Art and Design were at higher risk for mental health problems, while in the study by Odriozola-González et al. (43), students of Humanities and Arts and Social Sciences and Law appeared to be more vulnerable than students of Engineering and Architecture. A possible explanation for these results could be different difficulties and demands placed on students across various fields of study, which may have been reflected in their psychological distress.

Housing during the semester was also one of the important predictors of psychological problems during the COVID-19 pandemic. Living away from home was considered a risk factor for Internet addiction and depressive symptoms in both countries, and for anxiety symptoms in Slovak Republic. In these cases, students who lived away from home during the semester were more likely to have mental health problems compared to students who lived at home during the semester. This finding can be explained by distance from family support, infrequent contact, and less satisfaction with living in a dormitory (44, 45). In fact, parents and sufficient time spent with family members play an important role in students' wellbeing (44, 46). The results are consistent with those of Anand et al. (47), who found higher Internet addiction among students staying in rented accommodations. According to Romero-Rodríguez et al. (71), living outside the parents' house indicated higher rates of problematic Internet use, with parents at home showing a positive effect on reducing addictive online behaviors. Thus, the presence of parents at home may be an influential factor in terms of Internet addiction.

Regarding the distance between home and college, most of the analyzed cases were not significant. Slovak students who traveled between 50.1 and 100 km from home to college were less likely to be depressed compared to students who traveled more than 100 km. Also, Slovak students who traveled 20 km or less from home to college were less likely to be stressed than students traveling more than 100 km. Overall, the stated hypotheses H2a – H2e could be partially supported.

In the light of the presented findings, the need for increased mental health monitoring of higher education students can be emphasized. In this case, it appears crucial to identify effective interventions aimed at improving the mental health of Czech and Slovak youth during a difficult period in their lives, such as the COVID-19 pandemic. Professionals and policy-makers in both countries should therefore develop and implement strategies to effectively prevent mental health problems, including Internet addiction, anxiety, depression, as well as perceived stress. Overall, campaigns and programs should focus on full-time students and students living away from home during the semester. Problematic Internet use needs particular attention, as Internet addiction has been found to be associated with other problems that can further impair the mental state of young people. Therefore, efforts to help students addicted to the Internet are more than desirable. In this respect, the results showed that attention should be focused on full-time students, students living away from home during the semester, and students in Informatics, Mathematics, and ICT. With a reduction of Internet addiction, improvements in mental health in terms of anxiety symptoms, depressive symptoms, and stress can also be expected. Internet use should also be monitored during distance learning, when parents play an important role.

To improve the situation not only during the pandemic, a college-wide approach to students' mental health is needed in both countries, with teachers, assistants, professionals, and parents, but also policy-makers play an important role. It is crucial that Czech and Slovak policy-makers realize the importance of investing sufficient resources in special education and support for mental health care. In this sense, they should focus on strengthening the provision of early interventions and the development of open access mental health services for young people. Colleges and universities themselves should have the right conditions in place, including accessible counseling and equipping teachers with standardized mental health interviews to identify who needs help. Several colleges already have their own support centers that do activities to prevent and help students who need it. However, there are still colleges that do not provide such assistance to students or, on the other hand, do not have the capacity to help everyone. If a college does not have its own capacity, it is still possible to refer students to other organizations that provide this service, even virtually. Internet interventions in particular seem to have great potential in addressing this problem among today's college students. In this sense, a threat can turn into a help. The key is for colleges to communicate these opportunities. In the future, it is essential to collaborate more and help to improve the mental wellbeing of students studying in Czech Republic and Slovak Republic. Last but not least, the connections between colleges and families need to be improved. All of these efforts can lead to more intensive help, reduced stigma, and improved mental health literacy. In this respect, the COVID-19 pandemic can be seen as an opportunity to address a hitherto overlooked problem.

The strengths of the study include in particular the large research sample in both countries (given their size) and the coverage of almost all universities and colleges, which increases the credibility of the results. This study also offers an in-depth insight into a serious social problem not only during the pandemic in the two countries that were directly compared. At this point, it should be emphasized that during the pandemic, most Slovak and Czech colleges opted for distance learning, which may increase the likelihood of internet addiction. Therefore, there is a need for research in this area to contribute to the knowledge and understanding of this phenomenon not only in the individual phases of the pandemic but also after the pandemic. Despite the importance of the issue, there is a lack of Slovak and Czech studies focusing on Internet addiction and its relationship with other mental health problems. Thus, this study contributes to addressing the limitations in the current literature by providing valuable findings and a better understanding of the issue. This can be seen as a strength from both a theoretical and practical point of view.

The study did not avoid certain limitations. The first limitation was that the approach to dealing with the pandemic in a higher education environment was not identical in the examined countries. Moreover, it was not the same even within individual colleges in one country. This may have created different conditions for students. In order to eliminate the risk of bias in the results due to different pandemic measures in the countries, the analyses were performed separately for Czech Republic and Slovak Republic. Sorting the analyses by individual colleges would not be practical, but although this step has not been taken, no significant bias in the data is expected. The second potential limitation was a certain disproportionality of the sample, as well as a lower number of observations in some categories. In this context, it should be noted that obtaining an ideal sample during a pandemic is considerably challenging. This is also due to the online survey, which has some limitations in capturing respondents. Given the situation, efforts have been made to obtain as representative a sample as possible. Regarding the lower number of observations, more caution is needed in interpreting the results, especially those related to the study field. Future research ambitions should focus on comparing the pandemic and post-pandemic periods in terms of the mental health problems of higher education students. Future research should also cover more countries.