This study will examine the effect of bright light or melatonin treatment on sleep in children with Smith-Magenis syndrome (SMS), a genetic disorder characterized by certain physical, behavioral and developmental features. Patients have a disrupted sleep cycle involving early waking, frequent daytime napping and frequent nighttime awakenings. Melatonin is a hormone normally produced at night in healthy people. People with SMS produce high levels of melatonin during the daytime and very low levels at night. This may affect their behavior, mood, attention span and sleep patterns.