Background and aims: IDeg is a basal insulin with a long duration of action and a flat glucose-lowering profile. The risk of hypoglycaemia with IDeg vs IGlar U100 was compared in 2 trials in patients with T1D (SWITCH 1) or T2D (SWITCH 2) using a range of clinically relevant definitions of hypoglycaemia. Materials and methods: Two 2×32-week, double-blind, TTT, crossover trials compared the risk of overall (severe [requiring third-party assistance and external adjudication] or blood glucose [<3.1 mmol/L (56 mg/dL)] confirmed) symptomatic hypoglycaemia, nocturnal (severe or BG confirmed; between 00:01 and 05:59 am) symptomatic hypoglycaemia and severe hypoglycaemia with IDeg once daily vs IGlar U100 OD. Post hoc analyses investigated the risk of asymptomatic hypoglycaemic episodes, hypoglycaemia during sleep, and the American Diabetes Association definition of symptomatic confirmed (BG <=3.9 mmol/L [70.2 mg/dL]) hypoglycaemia between treatments. All endpoints were analysed in the 16-week maintenance period and the full treatment period (32 weeks). Results: HbA1c non-inferiority was confirmed in both trials. SWITCH 1 and SWITCH 2 had significant reductions in overall symptomatic hypoglycaemia and nocturnal symptomatic hypoglycaemia in both the maintenance and the full treatment periods with IDeg vs IGlar U100. SWITCH 1 had a significant reduction in severe hypoglycaemia with IDeg in the maintenance and the full treatment periods; in SWITCH 2 the rate ratio of severe hypoglycaemia was significantly lower with IDeg in the full treatment period. Significant risk reductions with IDeg vs IGlar U100 were found in the maintenance period for overall and nocturnal symptomatic or asymptomatic hypoglycaemia. When the definition of the nocturnal period was expanded to 10:01 pm-07:59 am, rates of nocturnal symptomatic hypoglycaemia during sleep were again significantly lower with IDeg vs IGlar U100 in the maintenance period. The finding of significantly lower rates of hypoglycaemia in the maintenance period with IDeg was retained when the ADA definition of symptomatic hypoglycaemia was used. Conclusion: Hypoglycaemia reductions were consistent across hypoglycaemia definitions, especially during the nocturnal period, in SWITCH 1 and SWITCH 2.