Study design and participants

This study was based on the health survey of the Great East Japan Earthquake (GEJE) survivors conducted every year since the GEJE in Miyagi Prefecture, Japan. The design of the health survey of the GEJE survivors has been described in detail elsewhere [17,18,19,20]. In brief, between July and August 2020, a self-reported questionnaire was administered to residents aged 18 years or older who were living in Ogatsu and Oshika, located in Ishinomaki City, which suffered extensive damage in the GEJE in Miyagi Prefecture, northeastern Japan (the health survey in 2020). The questionnaire included items on residents’ life environment, such as housing type, medical history, self-rated health, economic status, smoking habits, drinking habits, dietary habits, sleep disorder (based on the 8-item Athens Insomnia Scale [AIS]) [21], psychological distress (based on the 6-item Kessler Psychological Distress Scale [K6]) [22], working status, and social network (based on the 6-item Lubben Social Network Scale [LSNS-6]) [23, 24]. Three items about the effect of the COVID -19 pandemic (anxiety, frequency of going out, and income) were added to the health survey in 2020.

Of the 3,425 eligible subjects, 2,208 responded to the health survey in 2020 (response rate: 64.5%); 102 subjects who had missing questionnaire data for anxiety about the COVID-19 pandemic were excluded. Another 30 subjects who had missing questionnaire data for sleep disorders were also excluded. Thus, 1,976 subjects were included in the study cohort (Fig. 1).

Fig. 1Exposure variable: anxiety about the COVID-19 pandemic

Anxiety about the COVID-19 pandemic was assessed by responses to the following question: “How often do you feel anxious about the COVID-19 pandemic?”. The subjects were divided into four groups based on their responses to the question: “never”, “a little bit”, “sometimes”, and “usually or always”.

Outcome variable: sleep disorder

Sleep disorder was measured using the Athens Insomnia Scale (AIS) [21]. The AIS is an individual’s own assessment of any sleep disorder he/she might have experienced, provided that it occurred at least three times per week during the last month. It contains eight questions rated from 0 to 3, with a total score ranging from 0 to 24. A higher total AIS score indicates poorer of sleep status, and 6 or above indicates a sleep disorder [21].

Mediating variable: the decreased frequency of going out during the COVID-19 pandemic

The decreased frequency of going out during the COVID-19 pandemic was assessed by responses to the following question: “Did you change the frequency of going out due to the COVID-19 pandemic?”. The subjects were divided into four groups based on their responses to the question: “never changed”, “reduced by 20–30%”, “reduced by half”, and “hardly ever went out”.

Covariates

Self-rated health was evaluated by asking about health status (very good, good, poor, or bad). Self-rated health was categorized into two groups: “very good” and “good” or “poor” and “bad”. Economic status was evaluated by asking about current household economic status as assessed by degree of financial difficulty (very difficult, difficult, a little difficult, or normal). Economic status was categorized into two groups: “very difficult” and “difficult” or “a little difficult” and “normal”.

Statistical analysis

First, the characteristics of the study subjects at the health survey in 2020 were examined. Then, the mean AIS scores at the health survey in 2020 were examined across the anxiety groups. ANOVA was used to analyze the association between anxiety and mean AIS scores. Third, mediation analyses were performed for the association between anxiety and sleep disorder during the COVID-19 pandemic, with decreased frequency of going out as a potential mediating variable. Stata’s causal mediation analysis was used to estimate the standardized total, direct, and indirect effects and the proportion of the total effect mediated by the decreased frequency of going out after adjusting for covariates. Standard errors and confidence intervals were obtained using the bootstrapping method (1000 repetitions) [25].

The following variables were considered possible confounders: age (continuous variable), sex (male or female), self-rated health [good (very good or good), poor (poor or bad), or missing], and economic status [severe (very difficult or difficult), normal (a little difficult or normal), or missing]. These items were chosen as covariates because both health and economic status may affect the association between anxiety and sleep disorders.

All P values were two-sided, and differences at p < 0.05 were considered significant. Mediation analyses were conducted using Stata Statistical Software, version 16.0 (Stata Corp LLC, College Station, TX, USA). All other statistical analyses were performed using the SAS statistical software package, version 9.4 (SAS Institute Inc, Cary, NC, USA).

Ethical issues

The research related to human use has been complied with all the relevant national regulations, institutional policies and in accordance the tenets of the Helsinki Declaration. The study protocol was approved by the Institutional Review Board of Tohoku University Graduate School of Medicine (approved number: 2011–92, 2020–1-465). Consent to participate in the study was obtained from the participants either face-to-face or through the signed self-administered questionnaire.