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Background Soy isoflavone is beneficial for menopausal/postmenopausal symptoms, including sleep complaints. However, little is known about its longitudinal association with sleep in the general population. Objective Our aim was to investigate the association between soy isoflavone intake and sleep duration and daytime falling asleep among Chinese adults. Design A longitudinal analysis was performed. Soy isoflavone intake was assessed by food frequency questionnaire. Sleep duration was self-reported at two time points. Occurrence of daytime falling asleep was determined at follow-up. Short and long sleep were defined as sleep <7 h/day or ≥9 h/day, respectively. Participants/setting Adults aged 20 years and older from the Jiangsu Nutrition Study (2002-2007) with complete isoflavone intake and sleep duration data at both time points (n=1,474) were analyzed (follow-up, n=1,492). Main outcome measures We measured sleep duration in 2002 and 2007 and daytime falling asleep occurrence in 2007. Statistical analyses performed Mixed-effects logistic regression was performed for repeated measures between isoflavone intake and sleep duration. Logistic regression was performed for daytime falling asleep at follow-up. Demographic, anthropometric, and social factors were adjusted in the analyses. Results The prevalence of long sleep duration was 18.9% in 2002 and 12.6% in 2007, and the prevalence of daytime falling asleep was 5.3%. Compared with the lowest quartile of isoflavone intake, the highest quartile was associated with a lower risk of long sleep duration (odds ratio=0.66; 95% CI 0.48 to 0.90; P for trend=0.018) over 5 years. Compared with persistent low intake of isoflavone (less than median intake of isoflavone at two time points), persistent high intake was associated with a reduced risk of daytime falling asleep in women (odds ratio=0.20; 95% CI 0.06 to 0.68), but not men. No consistent association between soy isoflavone intake and short sleep duration was found. Conclusions Soy isoflavone intake was associated with a low risk of long sleep duration in both sexes and a low risk of daytime falling asleep in women but not men. [ABSTRACT FROM AUTHOR]