After you eat, your body works to digest what you’ve eaten. Since your body’s energy is directed towards your stomach, this leaves less energy for other organs, including your brain.
This is why your thoughts are less clear after a meal than before one. The heavier the meal, the lower the quality of your post-meal thinking.
As a result, if you have something important to do, it’s best to do it on an empty stomach.
There is, however, a limit to this approach. Specifically, this doesn’t mean that you should starve yourself. If your stomach is too empty, this also prevents you from thinking well because you don’t have any energy for your body to direct towards your brain.
The solution is to keep a relatively empty stomach so long as you’re doing important things. And if you enjoy larger meals, reserve them for times when you don’t have something important to do.