All sorts of things can affect your brain’s ability to function. If you’re tired it can be a struggle to think, if you have had coffee or other caffeine it may seem easier to think. Most of this is to do with levels of chemicals in your brain.
Someone else may know other reasons why your brain works better at certain times – anyone?
As Suzi says, lots of things can affect how well your brain works and the level of different chemicals in your brain is a key one. The levels of these chemicals change in response to the food you eat, the amount of daylight you are exposed to, the amount of rest you have and other things.
My brain usually works best in the morning. How about you?