BY MOUTH: View Sources. 1  For insomnia: 2  0.3-5 mg at bedtime is a typical dose. 3  In children with insomnia due to delayed sleep onset, melatonin 5 mg at 6:00 PM daily. 4  In children with developmental disorders (including cerebral palsy, autism, and mental retardation), melatonin 5 mg at 8:00 PM daily. For insomnia: 2  0.3-5 mg at bedtime is a typical dose. 3  In children with insomnia due to delayed sleep onset, melatonin 5 mg at 6:00 PM daily.