This study evaluated the efficacy of ramelteon for insomnia in adult subjects with ADHD. Experimental Design: For this randomized, double-blind, placebo-controlled crossover trial, 8 mg of ramelteon was given nightly, within three hours of bedtime, to ADHD-insomnia subjects confirmed by DSM-IV-TR, ADHD-RS, MINI, and clinical interview). All subjects underwent two weeks each of ramelteon and placebo. Objective sleep measures were obtained by actigraphy. Subjective measures included: the Epworth Sleepiness Scale (ESS) and ADHD-RS. Principal observations: Of 36 subjects entering the study, 58% met criteria for circadian rhythm sleep disorder (CRSD), delayed sleep phase type. During ramelteon period, mid-sleep time, an indicator of circadian phase, occurred significantly earlier, by ~45 minutes compared to placebo period. An association was noted between the magnitude of the sleep phase advance and the timing of ramelteon administration in relationship to sleep start time, but did not reach statistical significance; maximal efficacy was noted 1.5 hours before bedtime. Paradoxically, ramelteon marginally, but significantly increased sleep fragmentation and ESS scores compared to the placebo state. Conclusions: Ramelteon is efficacious in maintaining an earlier sleep/wake cycle in adults with ADHD and CRSD but can have paradoxical fragmenting effects on sleep and exacerbate daytime sleepiness. In the presence of a circadian rhythm disorder, the usual dosing and timing parameters for ramelteon need to be carefully considered.