This study examined sleep disorder in older adults in terms of its association with daily physical activity and social relationships, both individually and in combination with each other. Our study showed that a high level of physical activity and a wide range of social relationships may, independently of each other, positively affect sleep quality (Table 2). This result is similar to previous findings that low levels of physical activity and narrow ranges of social relationships are related to poor sleep quality in older adults [9, 20]; it should be noted that, unlike these previous studies, we adjusted for the effect of one variable when measuring the other. However, the prevalence of sleep disorder was significantly lower only in the active group with a wide range of social relationships, which featured high levels of both variables, compared to the inactive group with a narrow range of social relationships (Table 3). These results do not support our hypothesis that high levels of one variable with low levels of the other (e.g., inactive people with a wide range of social relationships) results in a lower prevalence of sleep disorder than low activity levels and a narrow range of social relationships.
Sleep disorder and depressive syndrome are strongly linked , and the rates of both of these are significantly higher in older adults than in younger people . Overall, 83% of patients with depression have experienced some form of sleep disturbance  and, although the effects of daily sleep quality on next-day mood are larger than the effects of daily mood on sleep quality, sleep quality and mood share a bilateral relationship . Some previous studies have shown that strategies for regulating mood control can improve sleep quality . In this study, although no significant differences were observed, the groups with low levels on one variable (i.e., the inactive group with a wide range of social relationships and the active group with a narrow range of social relationships) showed 14–26% lower depressive syndrome scores (i.e., GDS score; Table 1) than the inactive group with a narrow range of social relationships. Meanwhile, the active group with a wide range of social relationships showed a significantly lower value than the inactive group with a narrow range of social relationships (37% lower) and the active group with a narrow range of social relationships (27% lower). The significant difference in sleep disorder between the inactive group with a narrow range of social relationships and the active group with a narrow range of social relationships disappeared after full adjustment for covariates, including depressive syndrome. However, the difference between the inactive group with a narrow range of social relationships and the active group with a wide range of social relationships remained significant (Table 3).
Researchers have highlighted several potential mechanisms by which physical activity affects sleep quality, including energy conservation, increases body and central nervous system temperatures, and reduces anxiety [35–37]. Meanwhile, the effects of social relationships on sleep quality are not fully understood. Theoretically, however, strong social relationships provide an evolutionarily adaptive function, creating a safe environment in which sleeping persons feel they are protected by others from dangers (e.g., in an evolutionary context, predators and enemies) . Recent studies have also hypothesized that social isolation causes chronic stress , directly affects anxiety, and decreases sleep satisfaction [20, 21]. Social isolation is related to lower physiological functioning and higher risks of physical disorders . In particular, addressing deficits in terms of social relationships and physiological activity may directly arrest early progression toward chronic diseases, and could also delay disease onset and lessen disease burden in later life . The above findings suggest that physical activity and social relationships produce overlapping, synergistic anti-anxiety and anti-depressant effects.
In a previous study, the present authors revealed that communal exercise has a lower hazard ratio for five-year morality than no exercise . The present study suggests that physical activity and exercising with others [39, 40] may have a synergistic effect on not only physical functions but also mental health and sleep quality in older adults.
A previous epidemiological study showed that intellectual, social, and recreational activities are correlated with cognitive function . Indeed, an experimental study revealed that low-intensity exercise with social interaction not only has positive effects on sleep quality but also improves cognitive function, particularly attention ability, in older adults . It has been postulated that exposure to social relationships and engaging in physical activity increase blood flow to the brain, improving neuronal function and facilitating deep sleep [22, 23]. We suggest that future studies should examine whether the effects of physical activity and social interaction on older adults’ sleep quality are mediated by improved cognitive function; methods such as polysomnography and functional near-infrared spectroscopy could be used to perform this examination.
Our study has several limitations. First, although this study examined the lower prevalence of sleep disorder among individuals who are active and have a wide range of social relationships (i.e., lacking in neither variable), we could not explain the causal relationship because of the cross-sectional study design applied. Thus, this aspect warrants further investigation through a prospective cohort research or an intervention study, for example. Second, we did not examine whether supportive and aversive social relationships have different effects on sleep disorder. In this study, we only focused on supportive relationships because a previous study revealed no significant association between aversive relationships and sleep quality . Nevertheless, further studies should consider adverse relationships in this regard. Third, all of the variables were assessed using self-report scales; thus, our findings may have been influenced by recall and reporting bias. Fourth, physical activity and social relationship factors may be influenced by individual differences. Thus, future studies should consider gender differences and implement stratification in terms of physical function and the presence of depressive syndrome. Finally, although a previous epidemiological study involving a Japanese population  reported the same prevalence of sleep disorder (36.2%) as in the present study, the generalizability of our results is uncertain because of the small sample size.