So many people have gone on wild goose chases for supplements and accomplished nothing good. I always suggest developing good sleep hygiene before trying supplements or drugs. A good resource is http://www.umm.edu/sleep/sleep_hyg.htm .
Exercise and proper diet may be the most important factors that people most often don't take care of regularly.
Of course always use your CPAP and check the data to make sure your CPAP therapy is effective.
My doctor tested me and found low levels of Vitamin D which I will start taking prescription Vitamin D soon. She said it should help with the fatigue and sleeping.