ZT

You eat carbohydrates (starch is a carb). In response, your body creates insulin, which is a hormone which tells your body that you're about to recieve a dose of blood sugar (which is what digested starch turns into). In response to the insulin, your body takes all of the sugars that are already in your blood, and moves them into your cells to be burned for energy. (Makes sense, since you know you're about to get more, right?)

That makes you tired shortly after eating starch. But that's not all.

Several hours later, unless you keep eating sugary/starchy foods, your body has to make the transition from burning your available blood sugar to burning fat for energy. That transition is very hard for your body to make, as it wants to keep burning sugar. Before it makes the change, it will try for a good half-to-three hours to burn blood sugar that's not there -- and that makes you sleepy.