Association between obesity and self-reported sleep duration variability, sleep timing, and age in the Japanese population.

Objective: The objective of this study was to investigate the association between obesity and sleep habits, including bedtime, wake-up time, and sleep duration variability over a week.

Methods: We analyzed 9289 participants in this study. Following a health examination, each participant self-reported their sleep habits using a standardized 19-item questionnaire. High sleep duration variability was defined as sleep varying ≥3h of the difference between the longest and shortest sleep durations reported over a week-long period. Late bedtime and early wake-up time were defined as 24:00h or later, and before 6:00h, respectively. Obesity was defined as a body mass index (BMI) of ≥25kg/m. Subgroup analysis included age (younger group defined as age <65 years vs. elderly group defined as age ≥65 years).

Results: After adjusting for confounding factors and sleep duration, high sleep duration variability [odds ratio (OR), 1.20; p<0.005] was significantly associated with a high OR of obesity. Late bedtime (OR, 1.18; p<0.01) was significantly associated with a high OR of obesity, whereas early wake-up was not. In the subgroup analysis, high sleep duration variability had a significant association with the younger group (OR, 1.25; p<0.001), whereas late bedtime had a significant association with the elderly group (OR, 1.36; p<0.005).

Conclusions: Inappropriate sleep habits, particularly high sleep duration variability in young individuals and late bedtime in the elderly, are associated with a high prevalence of obesity, independently of sleep duration.