Our study revealed that long sleep duration and physical inactivity were independently associated with the risk of incident disability among older adults. These results were sustained after adjusted for age, sex, BMI, education, medication, medical history, current drinking habit, current smoking habit, GDS score and MMSE score. Coexistence of long sleep duration and physical inactivity had a higher risk of incident disability than each factor individually, even though the synergistic interaction was mildly effective.
In this study, long sleep duration was shown to be associated with incident disability during a 5-year follow-up period. This result is consistent with our previous study conducted among other participants during a 2-year follow-up period.[15] Other previous cross-sectional studies showed that long sleep duration was also associated with other health adverse outcomes, such as physical frailty[29] and social frailty.[30] Additionally, long sleep duration was a risk factor for cognitive decline after 4 years among older adults.[31] These functional declines are included in geriatric syndromes and are serious risk factors for incident disability.[32–34] Although our study could not examine the cause of incident disability in detail, these functional declines would be mediated by the association between long sleep duration and incident disability. Furthermore, long sleepers were also found to have an increased wake after sleep onset (WASO) frequency and lower sleep efficiency.[35] Longitudinal study revealed that sleep complaints and lower sleep efficiency were associated with incident ADL disability among older adults.[4] [36] Therefore, the poor sleep quality would also be an underlying factor behind the association between long sleep duration and incident disability.
Physical inactivity was also independently associated with incident disability in this study. In older adults, physical activity contributes to the prevention of disability caused by chronic diseases or functional decline.[37–39] Physical inactivity was associated with incident disability over a 9-year period among older adults.[40] Our result was in line with these previous studies. IPAQ-SF, which was used to assess physical activity in our study and allows us to assess physical activity and classify it according to three levels, can be easily used in clinical or research settings with lower respondent burden. Previous studies reported that physical activity assessed by IPAQ-SF has a cross-sectional association with health-related outcomes including quality of life, metabolic syndrome, sarcopenia and mental health.[41–44] These studies would support the association between IPAQ-assessed physical inactivity and incident disability.
Coexistence of long sleep duration and physical inactivity showed a higher hazard ratio of incident disability than each factor individually. However, RERI between long sleep duration and low physical activity with respect to incident disability was small, indicating that the synergistic interaction was mildly effective. The effects of long sleep duration and physical inactivity on incident disability would be not synergistic but independent, although several studies have observed that long sleep duration is associated with a sedentary lifestyle and low levels of daytime physical activity.[16–18] Further studies would be needed to examine the association of all-day activity, including daytime sleep or other activities, with incident disability.
A major strength of this study was its use of a prospective cohort design, which can address the causality between sleep duration and incident disability among older adults using large cohort data. However, this study had several limitations. First, self-reported measures of sleep duration were utilized, rendering the study incapable of performing a truly objective assessment of sleep characteristics, such as an assessment performed using actigraphy. Previous research suggested that self-reported long sleepers,[45] as well as average sleepers,[46] tend to overestimate their total sleep time, instead considering their entire time in bed. Similarly, self-reports of physical activity are likely to overestimate time spent in all intensities of physical activity.[47] Further studies that assess sleep duration and physical activity objectively using devices might show different results from ours, as we used self-report questionnaires to assess sleep duration and physical activity. Second, there were other potential confounders that we could not assess, such as drug therapy. Third, medical diagnoses were obtained by self-report, not by consulting medical records. Furthermore, given the absence of random sampling methods, our participants were exclusively those who had the ability to receive health checkups from their homes. Therefore, our results may not be directly applicable to all older adults in Japan, which is one of the limitations of this study.