Sleep disruptions and circadian sleep disruptions are integral to the clinical presentation of both PD and DLB. As is true in HD, sleep disturbances in PD and DLB cause severe disruption to the patients and their caregivers' lives. In PD, sleep dysfunction occurs in approximately two thirds of patients. Sleep problems range from difficulty with sleep initiation, sleep fragmentation, disturbance of circadian rhythm, REM sleep behavior disorder (RBD), to excessive daytime sleepiness. Frequent nighttime awakening and sleep disruption are the most common sleep problems in PD. In DLB, REM sleep behavior disorder (RBD) occurs years to decades before the onset of dementia. Importantly, melatonin is one of the main treatments used for RBD. Therefore, a melatonin agonist such as Ramelteon is a natural choice for the treatment of circadian sleep disturbances in PD and DLB.