Fourteen inpatients with dementia showing sleep and behavior disorders (average age = 75 years), and 10 control elderly people (average age = 75 years) were carefully observed for 2 months. Four weeks of morning light therapy markedly improved sleep and behavior disorders in the dementia group. The measurement of sleep time and the serum melatonin values suggests that sleep and behavior disorders in the dementia group are related to decreases in the amplitude of the sleep-wake rhythm and decreases in the levels of melatonin secretions. Morning light therapy significantly increased total and nocturnal sleep time and significantly decreased daytime sleep time. These results indicate that morning bright light is a powerful synchronizer that can normalize disturbed sleep and substantially reduce the frequency of behavior disorders in elderly people with dementia.