Cross-sectional Shika study links poorer sleep efficiency with distress only among working older women

In a cross-sectional analysis of 360 adults aged 40 to 90 in Shika Town, Japan, lower objectively measured sleep efficiency was associated with greater psychological distress only among women who were working. The association remained significant after adjustment for several confounders, while no significant relationship was detected among men or nonworking women. Because the data were collected at one point in time, the findings cannot establish whether poor sleep contributes to distress or whether distress and work circumstances affect sleep.

The study used data from the Shika study, an ongoing investigation of residents in four districts of Shika, a rural town in Ishikawa Prefecture. Investigators invited 550 residents older than 40 to participate, and 437, or 80%, took part. The final analysis excluded 28 participants without fasting-glucose data, 40 with a history of insomnia, sleep apnea, another sleep disorder or mental illness, and nine with a history of using psychotropic drugs or sleeping pills. The resulting sample included 164 men and 196 women; women represented 54.4% of the cohort, and the overall mean age was 63.09 years with a standard deviation of 10.48 years.

Participants wore a MTI-220 actigraphy device on the waist for one week to record sleep activity. They also maintained a sleep log of bedtimes and waking times. Actigraphy provided total sleep time, sleep latency, sleep efficiency and positional changes, while the logs supplied time in bed. Sleep efficiency was calculated as total sleep time divided by time in bed, multiplied by 100. Poor sleep efficiency was defined as less than 80%, and good sleep efficiency as at least 80%. The one-week measurement window and the reliance on self-reported time in bed limited the objectivity of the efficiency measure.

Psychological distress was screened with the Japanese Kessler 6 scale, or K6. Its six items ask how often respondents felt nervous, hopeless, restless or fidgety, depressed, as though everything was an effort, and worthless during the preceding 30 days. Responses range from 0 to 4 per item, producing a total score from 0 to 24, with higher values indicating greater distress. The researchers classified participants with K6 scores of at least 5 as having high psychological distress. They separately measured the frequency and level of stress with single-item questions.

Two-way analysis of covariance examined whether sleep-efficiency group and work status interacted in relation to K6 and the two stress measures, with adjustment for age, body mass index, smoking, alcohol consumption, education and step count. Among women, there was a significant interaction between sleep efficiency and work status for K6 scores, with an interaction p-value of 0.012 and partial eta squared of 0.034. Working women with poor sleep efficiency had higher mean K6 scores than working women with good efficiency in a Bonferroni comparison, while the two efficiency groups did not differ significantly among nonworking women. No significant interaction was found for stress frequency or stress level in either sex.

Multiple logistic regression treated membership in the high-K6 group as the outcome and included sleep efficiency among the explanatory variables, with models stratified by sex and work status. In women who were working, sleep efficiency was a significant negative predictor in the model adjusted for age and body mass index, with a coefficient of -0.060 and p=0.013. It remained a significant negative predictor after additional sets of confounders were introduced. The researchers found no significant explanatory variable for high psychological distress in men with or without work, nor in women who were not working.

The results were consistent with an association confined to working women in this sample, rather than a general relationship between sleep efficiency and distress across all participants. Women also reported higher stress frequency and stress levels than men, and women in the poor-sleep-efficiency group had higher mean K6 scores than women with good efficiency, including after adjustment for age. The study did not collect information that could fully evaluate several possible influences, including marital status, child-rearing, caregiving, significant life events, workplace position, length of service, rotating shifts, work-related physical activity and menopause.

Several additional limitations constrain the findings. Excluding people with prior sleep disorders, psychiatric conditions or relevant medication use may have produced selection bias and narrowed the population represented by the results. The sample was also small, particularly for subgroup analyses, and investigators did not estimate odds ratios for clinically meaningful increments in sleep efficiency. A one-week actigraphy period may not capture usual sleep patterns for people with variable schedules, while self-reported questionnaires and sleep logs may introduce recall or reporting bias. Finally, the participants came from one local town, where women’s employment was concentrated in professional and technical, service and clerical occupations, limiting generalization to other regions, countries or occupational groups.

Although the authors proposed that attending to sleep among middle-aged and older working women may be a potential strategy for reducing psychological distress, the study did not test an intervention or demonstrate a clinical benefit. Its supported conclusion is narrower: in this cross-sectional cohort, lower sleep efficiency was associated with higher psychological distress among working women, after adjustment for measured confounders. Longitudinal research incorporating more complete occupational, family and personal information is needed to clarify the direction and mechanisms of that relationship and whether it holds in broader populations.

Subject of Research: Sleep efficiency and psychological distress among middle-aged and older adults by sex and work status

Article Title: Relationship between objective sleep efficiency and psychological distress in middle-aged and older Japanese: Results by work in the Shika study

Article References: Sato, K., Tsujiguchi, H., Miyagi, S., Morita, E., Suzuki, F., Hara, A., Suzuki, K., Kannon, T., Nakamura, M., Takazawa, C., Tsuboi, H., Matsumoto, S., Ishihara, A., Hori, D., Doki, S., Sasahara, S.-I., Satoh, M., Matsuzaki, I., Yanagisawa, M., … Nakamura, H. (2026). Relationship between objective sleep efficiency and psychological distress in middle-aged and older Japanese: Results by work in the Shika study. BMC Psychology, 14(1), Article 1514. https://doi.org/10.1186/s40359-026-05656-2

Image Credits: AI Generated

DOI: 10.1186/s40359-026-05656-2

Keywords: sleep efficiency, psychological distress, Kessler 6 scale, actigraphy, working women, cross-sectional study, Shika study, Japanese adults, work status, Relationship, between, objective