I also have COSA. I am on C-Pap therapy and when I had been on therapy for about a month, I began feeling like that too. My doctor ordered an increase on the settings on my machine and that helped for a while. When I went in to my GP for a checkup, she found out that I also have a hypoactive thyroid gland and put me on a low dose of thyroid medication. When I explained about being on c-pap therapy and it not helping very much, she told me that it could be a combination of sleep apnea and hypothyroidism that is making me feel like that. I have been on the thyroid medication for 5 months now and I feel great. No more afternoon naps. That may not be your problem but I would go in for a check up to make sure that there is not another problem causing your afternoon tiredness. Good luck.