Secondary objective: (1) To study the effect of treatment with bright light to improve patient behavior disorders Alzheimer's disease, including its effect on sleep disorders (score obtained in item nocturnal sleep-behavior NPI -nurse, scale score of Cohen-Mansfield) ; (2) To study the effect of treatment with bright light treatment on cognitive function in patients with Alzheimer's disease (MMSE score of the scale). (3)To examine the interest of the wrist actigraphy to assess motor behavior and sleep in Alzheimer's disease.