Working out over lunch is something many of us do nowadays. Stressed as we are we try to fit in something to do whenever we can.
But you have to eat at lunch as well, after all, the lunch hour is for you to eat.
From CNN news

Three key elements come into play when replenishing the body post-workout: fluids, carbohydrates, and protein.

For you not to get tired after lunch a good lunch box is just what you need. You need to make sure your protein, fluids and carbohydrates are balances in a way that it benefits your body. After all you are working out at lunch to make sure you get time to do other things afterwards, not feeling tired. That is why it is extra important to eat well afterwards.

If you have time to work out at lunch, good for you. If you feel stressed then maybe it is not for you. Then either find another time to exercise on or reduce the amount of time you spend on exercising on your lunch hour.

Remember exercise is suppose to make you feel good!