Could be the food, but it's more likely how much you sleep at night that is causing this.
How much do you sleep at night on average?
There was a study done saying it isn't so much the time of day or the food as the fact that you have a sleep deficit going on and the food makes you a little tired, but because of the deficit you literally have to sleep.
If it is the sleep then sleep more, if it's not the sleep then don't feel bad about it. Look at it as a 10 minute power nap. They really are rejuventaing.