I think this is your problem:
Too heavy lunch with too much carbs.
Keep your lunch light with less carbs. Large amounts of carbs will make you sleepy about 1-2 hours after lunch. Happens to me in the office too.

It might also got nothing to do about sleepiness, it's just that your brains doesn't want you to study. So you get bored reading and revising. And your mind drifts away. -->Sleepy. X^D

Perhaps you'll need a different method for studying.
Writing short notes, drawing diagrams, strecth once a while, etc.