Alzheimer's disease and related syndromes (AD) is a disease affecting memory but also the relationship with the environment and empower people. Patients with AD present in 90% of cases of behavioral disorders and of these behavioral disorders include agitation, apathy but also sleep disorders by circadian rhythm impairment. In 2014, the Cochrane published a systematic review of the literature over the last 20 years concerning the use of light therapy in patients with a AD. No study has shown proven efficacy on the behavior of patients with AD. The main objective of the study was to assess the influence of environmental light therapy (from 5 a.m to 10 p.m) on nighttime sleep of residents. Secondary objectives were the study of sleep time on the day, anxiety by the COVI scale and behavioral disorders by the NPI scale.