Design and Participants
This study was a 5-year follow-up cohort study. Participants at baseline were community-dwelling older adults living in three areas of Ojiya City, Niigata, Japan. Among 592 residents aged ≥65 years receiving no long-term care insurance services who were invited to participate in the study, 535 (90.4%) underwent baseline examination. Of these, 509 participants who were considered cognitively normal were invited to participate in the present 5-year follow-up study, and 371 underwent follow-up examination. We also included 18 individuals who did not participate in the follow-up examination, but were diagnosed with dementia at medical facilities. The final study cohort thus comprised 389 individuals. Figure 1 shows the flow of participant enrollment. Informed consent was obtained from all participants. The consent was verbal because, according to the Ethical Guidelines for Medical and Health Research Involving Human Subjects in Japan , investigators are not required to obtain informed consent in writing for human studies which are not invasive or do not involve interventions. The protocol of this study was approved by the Ethics Committee of Niigata University.
The baseline examination was conducted in three areas of Ojiya city in 2011 (Heisei-cho), 2012 (Matto), and 2013 (Katakai). Trained nurses visited and interviewed participants to collect the following information: demographic characteristics, health status (including cognitive function) and lifestyle, family environment (living with family or alone), current occupational status (unemployed or employed), and histories of hypertension, cerebrovascular disease, and diabetes. Cognitive function was assessed using the revised Hasegawa’s dementia scale (HDS-R) . We also collected information regarding alcohol consumption (classified into five categories: 1) non-drinker, 2) <7 gou (1 gou is equivalent to 180 mL of Japanese sake), 3) 7-13 gou, 4) 14-20 gou, and 5) ≥21 gou per week) and smoking status (classified into three categories: 1) non-smoker, 2) past smoker, and 3) current smoker). Usual bedtime and sleeping hours were asked and recorded, and the duration of daytime nap was recorded as 1) none, 2) <30 min, 3) 30-59 min, and 4) ≥60 min. The presence of current sleep disturbance and use of sleeping pills were also asked. Details of the baseline examination have been described previously .
Five-year follow-up examination
The follow-up examination, including the HDS-R test, was conducted in 2016 (Heisei-cho), 2017 (Matto in), and 2018 (Katakai), in the same manner as in the baseline examination.
Assessment of cognitive function
The HDS-R, a 30-point test, was used to assess general cognitive function, with a score ≤20 defined as cognitive impairment at baseline. The HDS-R was developed to screen for dementia (sensitivity: 0.90, specificity: 0.82) with a cut-off point of 20/21 . The HDS-R has been used in East Asian populations [15,16] and demonstrated to have a diagnostic accuracy similar to that of the MMSE . One advantage of using the HDS-R over the MMSE is its diagnostic accuracy regardless of education level . The HDS-R was administered during baseline and follow-up examinations, and change in HDS-R (DHDS-R = [score at follow-up] – [score at the baseline]) was calculated. We used a cutoff of DHDS-R ≤-3 to detect the presence of cognitive decline, referring to the cutoff of DMMSE ≤-3 (30-point test) used in previous studies [18,19], based on the fact that longitudinal scores of HDS-R and MMSE change in the same direction in community-dwelling individuals . We also considered 18 individuals who had normal cognitive function at baseline and did not participate in the follow-up examination, but were diagnosed with dementia at medical facilities, as having cognitive decline.
The χ2 test was used to test for independence of categorical data for participant characteristics. Cumulative incidence of cognitive decline was calculated and compared according to levels of potential predictor variables by odds ratios (ORs) computed using simple and multiple logistic regression analysis. First, unadjusted, and age- and baseline-HDS-R-adjusted ORs for cognitive decline according to potential predictors were calculated. Second, ORs for cognitive decline according to bedtime, duration of sleep, and duration of nap were calculated, adjusted for age, baseline HDS-R, sex, region (dummy variables), family environment, job status, histories of hypertension, cerebrovascular diseases, and diabetes, alcohol consumption, smoking status, bedtime, duration of sleep, duration of nap (0, 1-29 min and 1, others), presence of sleep disturbance, and use of sleeping pills. SAS statistical software (release 9.1.3, SAS Institute Inc., Cary, NC, USA) was used for data analysis. P<0.05 was considered statistically significant.