Try a high protein low carb lunch. I have impaired glucose tolerance, and before figuring that out, would crash on my office floor for an afternoon nap. Same thing would happen if my thyroid hormone dosage was off (Hashimotos).

If you aren't sleeping, avoid stimulants like caffeine in the afternoon and evening, and see if that helps. If I have coffee too late in the day I won't sleep, even decaf!!