Part of our circadian rythm is affected by melatonin, which we produce naturally. Supplemental melatonin may help in "getting" to sleep, but may not assist in the QUALITY of sleep. Many that take sleep aids "get" sleep, but not RESTFUL sleep.
If you have sleep apnea, you MUST maintain the airway patency (open airway). CPAP does a good job, but oral appliances can help too. Melatonin will NOT assist with the airway.