ABSTRACT Aims/Introduction Previous observational studies have suggested an increased risk of type 2 diabetes associated with both short and long sleep duration. However, there remains uncertainty, particularly regarding the adverse effects of long sleep duration. We investigated the association between self‐reported questionnaire‐based and objectively measured accelerometer‐derived sleep duration and the risk of type 2 diabetes using data from the UK Biobank. Materials and Methods First, we performed conventional Cox regression analysis with restricted cubic splines to illustrate the potentially non‐linear association between sleep duration and the risk of type 2 diabetes. Second, we performed non‐linear Mendelian randomization (MR) analysis using the doubly‐ranked method with 85 and 20 genetic variants associated with questionnaire‐based and accelerometer‐based sleep duration, respectively. Third, we performed two‐sample MR analysis. Results The results of conventional analysis of accelerometer‐derived sleep duration did not suggest a strong association between longer sleep duration and type 2 diabetes risk (hazard ratio [HR] of ≥10 h compared with 7–8 h, 1.08; 95% confidence interval [CI], 0.92–1.27). The results of non‐linear MR showed no strong evidence for an increased risk of type 2 diabetes associated with questionnaire‐based longer sleep duration (HR of 9 h compared with 7 h, 0.77; 95% CI, 0.52–1.15). This finding was consistent with non‐linear MR of accelerometer‐derived sleep duration (HR of 9 h compared with 7 h, 0.78; 95% CI, 0.29–2.06). Conclusions Our findings suggest that longer sleep duration does not play a major role in the development of type 2 diabetes.