Study Objectives: To investigate parameters of sleep, activity, and circadian rhythm, as well as the effects of methylphenidate on these variables, in adults with ADHD. Design: 1) Baseline group comparison; 2) Double blind, placebo-controlled, cross-over medication trial. Setting: Data collection took place during daily lives of participants. Participants: 39 normal controls and 33 adults with ADHD for baseline comparisons; 31 adults with ADHD in medication trial. Interventions: Treatment with placebo and methylphenidate during medication trial. Measurements and Results: Actigraphy and sleep log data were collected for 7 consecutive nights and days to obtain baseline values for ADHD and normal controls. Repeated measurements during placebo and methylphenidate treatment were conducted for the ADHD group. Actigraphic sleep estimates showed that ADHD subjects took longer to fall asleep, had lower sleep efficiency, and had shorter within-night periods of uninterrupted sleep. These findings were consistent with subjective complaints. Actigraphic measures of ADHD subjects showed continuously elevated daytime activity levels, resulting in a 24-hour pattern that was more stable and less variable than in controls. Methylphenidate led to a later bedtime, later sleep onset, and reduction in sleep duration. However, number and total duration of nocturnal awakenings decreased, while mean duration of within-night periods of uninterrupted sleep increased, indicating more consolidated sleep. Conclusions: Our data suggest that sleep problems are inherent in adults with ADHD and that methylphenidate reduced total sleep time but improved sleep quality by consolidating sleep.