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Few studies are currently available regarding the influence of sleep duration on glycemic control in diabetic patients. The objective of the current study was to examine the relationship between sleep duration, obesity, and the glycemic level in type 2 diabetic patients.A total of 4,870 Japanese type 2 diabetic patients aged ≥20 years were divided into six groups according to their self-reported sleep duration: less than 4.5 h, 4.5–5.4 h, 5.5–6.4 h, 6.5–7.4 h, 7.5–8.4 h, and more than 8.5 h. The associations of sleep duration with obesity and the HbA1c levels were examined in a cross-sectional manner.The HbA1c levels showed a quadratic association with sleep duration; namely, a shorter or longer sleep duration was associated with a higher level compared with a sleep duration of 6.5–7.4 h (P for quadratic trend <0.001). This association remained significant after adjusting for potential confounders, including the total energy intake and depressive symptoms. Furthermore, additional adjustments for obesity, which also showed a U-shaped relationship with sleep duration, did not attenuate the U-shaped sleep-HbA1c association. A significant interaction between sleep duration and age or the use of insulin was observed for the HbA1c levels.Sleep duration was shown to have U-shaped associations with obesity and the HbA1c levels in type 2 diabetic patients, independent of potential confounders, and therefore may be an important modifiable factor for the clinical management of patients with type 2 diabetes.