Background: We assessed whether overnight home use of automated closed loop insulin delivery (artificial pancreas) improves glucose control. Methods: We studied 24 adults with type 1 diabetes in a multicentre crossover study design comparing four weeks of overnight closed loop using a model predictive control algorithm to direct insulin delivery, with four weeks of insulin pump therapy in which participants used real-time display of continuous glucose monitoring independent of their pumps as control. Primary outcome was time when glucose was in the target range of 3•9 and 8•0mmol/l between midnight to 07:00. Analyses were by intention to treat. Trial registration ClinicalTrials.gov NCT01440140. Findings: Closed loop was utilised over median 8•3 (interquartile range 6•0, 9•6)hours on 555nights (86%). Proportion of time when overnight glucose was in target range was significantly higher during closed loop compared to control by 13•5% (95% CI, 7•3-19•7; p<0•001). Mean overnight glucose (8•2±0•9 vs. 9•0±1•3mmol/l; p=0•005) and time spent above target (44•3%±11•9 vs. 57•1%±15•6; p=0•001) were significantly lower during closed loop. Time spent below target was low and comparable [1•8%( 0•6, 3•6) vs. 2•1%(0•7, 3•9);p=0•28]. Lower mean overnight glucose was brought about by increased overnight insulin delivery [6•4 (4•5, 8•1) vs. 4•9 (3•7, 6•3)units;p<0•001) without changing the total daily insulin amount [34•5 (29•3, 48•4) vs. 35•4 (29•7, 45•2)units;p=0•32]. No severe hypoglycaemia episodes occurred during control period and two during closed loop not related to algorithm instructions. Interpretation: Unsupervised overnight closed loop at home is feasible and may improve glucose control in adults with type 1 diabetes.