There are disturbed circadian rhythm in Alzheimer's disease (AD) patients, which is related to the declined cognitive function. Our group has reported that disturbance of circadian rhythm and melatonin level was an early event in the process of AD, which accompanied with the progression of AD neuropathology. The combined treatment with melatonin and light could delay the impairment of AD cognitive function. In the present review, we will discuss the relationship between disturbed melatonin rhythm and AD.