Objective Patients with Alzheimer dementia often display poor sleep. Given that the disease is often associated with low endogenous levels of melatonin, exogenous melatonin administration may lead to improvements in sleep. Methods Randomized, placebo controlled study was carried out in 41 patients with probable Alzheimer disease. Melatonin (8. 5 mg immediate release and 1. 5 mg sustained release) (n =24) or placebo (n = 17) administered at 22:00 P. M. for 10 consecutive nights. The protocol sleep was measured continuously using actigraphy. Results There were no significant effects of melatonin,compared with placebo,on sleep and circadian rhythms. Conclusion This study failed to find a beneficial effect of exogenous melatonin.