90 children with an autism spectrum disorder between the ages of 24 to 50 months will be screened via polysomnogram to find 45 with a relative REM deficiency. This group will then be divided into two arms of drug versus placebo. Allowing for a 22 % drop out rate we expect 17 in each group to complete the study. Additionally, we will enroll 16 children with an ASD who do not meet criteria for relative REM deficiency in an open label arm to ascertain whether or not donepezil is beneficial to behavior in this group.