First off let me say that the human body naturally wants to sleep mid-afternoon, our workdays do not fit our natural cycles. However for most people this is not a serious problem.

As you have seen a doc already try the simple things first; 8 hours sleep is the recommend minimum, some people are quite happy on less, but you could try getting a bit more and see if that helps. You could also try an energy snack mid-afternoon such as a banana or something similar, these can work better than caffeine for some people. If still not joy and if you are seriously drowsy then as Pete says seek a second opinion from a specialist physician.

OIMO