Abstract Objective To determine whether potential exposure to natural light via windows is associated with reduced delirium burden in critically ill patients admitted to the ICU in a single room. Design Prospective single-center study. Setting Medical ICU of a university hospital, Paris, France. Patients Adult patients receiving invasive mechanical ventilation. Methods Consecutive patients admitted to a single room with (LIGHT group) or without (DARK group) exposure to natural light via windows were evaluated for delirium. The primary endpoint was the incidence of delirium. Main secondary endpoints included incidence of severe agitation intervened with antipsychotics and incidence of hallucinations. Results A total of 195 patients were included (LIGHT group: n = 110; DARK group: n = 85). The incidence of delirium was similar in the LIGHT group and the DARK group (64% vs. 71%; relative risk (RR) 0.89, 95% CI 0.73–1.09). Compared with the DARK group, patients from the LIGHT group were less likely to be intervened with antipsychotics for agitation episodes (13% vs. 25%; RR 0.52, 95% CI 0.27–0.98) and had less frequent hallucinations (11% vs. 22%; RR 0.49, 95% CI 0.24–0.98). In multivariate logistic regression analysis, natural light exposure was independently associated with a reduced risk of agitation episodes intervened with antipsychotics (adjusted odds ratio = 0.39; 95% CI 0.17–0.88). Conclusion Admission to a single room with potential exposure to natural light via windows was not associated with reduced delirium burden, as compared to admission to a single room without windows. However, natural light exposure was associated with a reduced risk of agitation episodes and hallucinations.