What should we eat if you want to get a productive idea at work?

I found a article on this site.
Most of the workers might get tired in the afternoon occasionally, and it is a universal fact. 77 percent of people who answered the recent survey said that they often eat a certain food if they knew those foods make thier brain more fresh. According  to a small British research, Darkchocolates and waters are good foods to stay our brain more productive.

I am also getting tired after the lunch time, although I usually do not eat anything to work my brain  more productively, I found that there are several reasons which foods makes our thinking more productive.
For example the news site said that Darkchocolates help our feel awake because of the ingredient called "flavanols". Moreover water is also good for us. I thought water is noot food, and it dose not make someone help or awake. So I am surprised that we can not focus on the job because of the dehydrate including our brain. So water supplemented our body everyday.

Do you usually do or eat something to keep awaking from sleepiness?
What do you do if you want to focus on your job?