After our emotional centers are done processing new info, the next brain part to receive the data is the front brain, or the prefrontal cortex. This is a bit like our own personal assistant: it handles motor function, memory, language, problem-solving, impulse regulation, social behavior, and a bunch of other cognitive skills. When the front brain is exhausted or depleted, we experience a weariness that prevents us from getting anything done.
This is known as ego depletion