This study aimed to assess the association between a vegetarian diet and sleep quality among Chinese healthy adults and explore potential contributing factors. A cross-sectional study was conducted with 280 vegetarians and 280 age- and sex-matched omnivores from Shanghai, China. The Pittsburgh Sleep Quality Index (PSQI) and the Central Depression Scale (CES-D) were used to assess sleep and depression condition, respectively. A validated semi-quantitative food frequency questionnaires (SQFFQ) was employed to assess dietary intakes, and body composition was measured with InBody720. Multi-linear regression and logistic regression analysis were performed for the data analysis. The sleep quality was significantly better in the vegetarians than in the omnivores (PSQI score: 2.80 ± 2.02 vs. 3.27 ± 1.90, p = 0.005). The proportion of vegetarians who reported self-satisfied sleep was also higher than that of the omnivores (84.6% vs. 76.1%, p = 0.011). However, after adjusted for the depression condition (CES-D scores), the difference in sleep quality between vegetarians and omnivores became insignificant (p = 0.053). Compared to omnivores, vegetarians had lower depression scores (CES-D: 9.37 ± 6.24 vs. 10.94 ± 7.00, p = 0.006). After controlling for confounding factors, there was positive association between depression condition and sleep quality (β = 0.106, 95%CI: 0.083 to 0.129, p < 0.001). Similarly, participants with better CES-D score had a lower risk of sleep disorders after controlling for the same confounding factors (OR = 1.109, 95%CI: 1.072 to 1.147, p < 0.001). Different contributing factors were reported in the vegetarian group and omnivore group. In conclusion, a vegetarian diet might improve sleep quality by moderating mental health, particularly depression condition.
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