The present study aimed to clarify the relationship between sleep disturbance in adolescents and IA. We found a relationship between adolescent sleep disturbance and IA in one prefecture in Japan and found that sleep disturbances are more prevalent in categories with higher YDQ scores. Furthermore, the results of the multivariate analysis revealed significantly higher adjusted ORs between categories with high YDQ and sleep disorders, which suggests a significant relationship between higher YDQ scores (3–4 or ≥5) and sleep disorders.
Despite the importance of adolescent sleep, sleep disturbance was present in more than half of the study participants. Furthermore, the high proportions of adolescents with sleep disturbances were in accordance with the results of recent studies of older adolescents using the PSQI [47, 48]. In this study, sleep disturbance was more frequent in boys who participated in school sports (or clubs), overall skipped meals, and had depressive moods (mental health). These results were similar to those of previous studies linking regular sleep habits to psychological and physical health . In addition, the prevalence of sleep disturbance was higher in private schools and for students with longer commutes. In general, private schools often have longer commute distances; therefore, it may be associated with reduced sleep time as shown previously . Previous studies have suggested that sleep quality is related to health and emotions among the youth individuals [14, 50]. Future longitudinal studies should examine changes over time for these and other variables to better understand the relationship between sleep disorders and Internet dependence.
In addition, The association between the YDQ scores and sleep disorders is similar to that reported by Bakken et al.  from a study among Norwegian adults, aged ≥ 16 years, where individuals with high YDQ scores had significantly higher prevalence rates of sleep disturbance than non-problematic Internet users. Furthermore, the present study found differences between sexes, with a higher sleep disturbance adjusted OR in girls with IA than in boys. This finding was similar to the results of Durkee et al. , who confirmed a significant relationship between insufficient sleep and IA in girl participants, which was in line with the findings of a previous study .
There are several possible mechanisms for the relationship between sleep disturbance and IA. First, it is possible that IA can cause sleep disturbances. Tan et al. reported that problematic Internet use could cause sleep disorders . Moreover, Chen et al. indicated that IA predicted a “disturbed circadian rhythm” leading to sleep disturbance , which is one explanation for the relationship between IA and sleep disturbance.
The second possible mechanism is the opposite of the one discussed above, whereby sleep disturbance might lead to the development of IA. In a longitudinal study, Chen et al. reported that falling asleep and nocturnal awakening difficulties were predictors of IA .
A third possible mechanism is that both conditions contribute to each other. Especially, sleep disturbance may contribute to IA, and IA may contribute to sleep disturbance. Several studies on adults have confirmed that, in brain imaging, sleep disturbance and IA cause changes in the gray matter [52, 53]. A study of retired military personnel showed that, regardless of any coincidental psychiatric state, individuals with a high PSQI score presented with reduced volume of the entire cortex and frontal lobes . Another study that did not control for sleep disturbance reported that individuals with IA reportedly had reduced gray matter density . These findings suggest the possibility that IA may cause organic (structural) changes in sleep-related neural pathways.
The following three points can be considered as the strengths of this study. First, the sample size was adequate to ensure statistical power. Second, to investigate the relationship between sleep disturbance and IA, we used the PSQI and YDQ, which have been frequently used as standard indices in several epidemiological surveys [5, 10, 25, 39, 41, 43, 51, 52, 54-59]. Third, in our analysis, we evaluated the relationships between sleep disturbance and IA for each of the three categories of the YDQ, including at-risk Internet use; this category has not been sufficiently investigated in previous epidemiological studies.
Nonetheless, the present study had a number of limitations. First, the present study analyzed with the results of a cross-sectional survey, which implies that we cannot formulate any conclusion regarding the direction of causality. Second, statistical analysis should have taken into account schools as cluster units. Third, we did not adjust ORs for all the items that may be related to IA. For example, we did not ask questions regarding other psychiatric disorders, such as attention deficit and hyperactivity disturbance (ADHD), which are reportedly associated with IA [60-62] and sleep disturbance  in adolescents. In the present study, all participants were enrolled as daytime high school students who regularly attended school. In such a scenario, the number of students with ADHD is considered low. Fourth, as we conducted the survey within each of the schools, non-attending students could not participate, and future surveys that will enable participation of non-attending students should be undertaken. Fifth, our survey population was limited to students in a single prefecture in Japan; thus, there was certainly a sampling bias. Finally, we did not investigate specific Internet-use disorders [4, 63-69], which need to be investigated in detail for preventive measures to be developed.