They already nailed it, but I agree with talking to your doctor for more definite information. I also agree with the sleep. I know, personally, I feel "rested" when I get 9 hours each night. (Typing that makes me laugh because I have a 9 month old son who doesn't sleep a wink at night so like last night I got about 3 and have NO time for naps during my days, but if I *could,* I would aim for 9. :) ) It's possible that for you to feel your best you may need more.
Also, it sounds like your diet is pretty balanced, but are there certain higher carb foods (maybe lacking protein) in the afternoon that makes you sleepy after eating?
And it also sounds like you keep yourself relatively busy...maybe you're just tired from the day! I think if I could take a nap in the afternoon, I'd do it regardless of sleep at night.
Which brings me back to checking with your doctor. It could be something else...it could be nothing, but your doctor can help you more! :)