Is there any reason of restlessness in the afternoon?

I have been practicing mindfulness and watching thoughts. I usually feel rapture, calm and desireless all the day but not in afternoon after lunch. In afternoon, I even forget all stability including mindfulness & various desires creeps in the mind. Situations become extraordinarily better after 4-4:30PM. Is there any particular reason of afternoon (2-4) instability? What can I do to let that not happen?