Abstract Aims/hypothesis This study aimed to describe the relationship between breastfeeding episodes and maternal glucose levels, and to assess whether this differs with closed-loop vs open-loop (sensor-augmented pump) insulin therapy. Methods Infant-feeding diaries were collected at 6 weeks, 12 weeks and 24 weeks postpartum in a trial of postpartum closed-loop use in 18 women with type 1 diabetes. Continuous glucose monitoring (CGM) data were used to identify maternal glucose patterns within the 3 h of breastfeeding episodes. Generalised mixed models adjusted for breastfeeding episodes in the same woman, repeat breastfeeding episodes, carbohydrate intake, infant age at time of feeding and early pregnancy HbA_1c. This was a secondary analysis of data collected during a randomised trial (ClinicalTrials.gov registration no. NCT04420728). Results CGM glucose remained above 3.9 mmol/l in the 3 h post-breastfeeding for 93% (397/427) of breastfeeding episodes. There was an overall decrease in glucose at nighttime within 3 h of breastfeeding (1.1 mmol l⁻¹ h⁻¹ decrease on average; p=0.009). A decrease in nighttime glucose was observed with open-loop therapy (1.2 ± 0.5 mmol/l) but was blunted with closed-loop therapy (0.4 ± 0.3 mmol/l; p<0.01, open-loop vs closed-loop). Conclusions/interpretation There is a small decrease in glucose after nighttime breastfeeding that usually does not result in maternal hypoglycaemia; this appears to be blunted with the use of closed-loop therapy. Graphical Abstract