Patients admitted for rehabilitation often lack sufficient natural light to entrain their circadian rhythm. Installed diurnal naturalistic light may positively influence the outcome of depressive mood, anxiety, and cognition in such patients. A quasi-randomized controlled trial. Ninety stroke patients in need of rehabilitation were randomized between May 1, 2014, and June 1, 2015 to either a rehabilitation unit equipped entirely with always on naturalistic lighting (IU), or to a rehabilitation unit with standard indoor lighting (CU).Examinations were performed at inclusion and discharge. The following changes were investigated: depressive mood based on the Hamilton Depression scale (HAM-D6) and Major Depression Inventory scale (MDI), anxiety based on the Hospital Anxiety and Depression Scale (HADS), cognition based on the Montreal Cognitive Assessment (MoCA) and well-being based on the Well-being Index (WHO-5). Depressive mood (MDI p = 0.0005, HAM-D6 p = 0.011) and anxiety (HADS anxiety p = 0.045) was reduced, and well-being (WHO-5 p = 0.046) was increased, in the IU at discharge compared to the CU. No difference was found in cognition (MoCA p = 0.969). This study is the first to demonstrate that exposure to naturalistic light during admission may significantly improve mental health in rehabilitation patients. Further studies are needed to confirm these findings.