system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens in our brains when we get sleepy from doing something boring to us versus doing something interesting that will keep us awake for several more hours?
gpt
Directed attention fatigue (DAF) is a neurological phenomenon that results from overuse of the brain’s inhibitory attention mechanisms, which handle incoming distractions while maintaining focus on a specific task. The greatest threat to a given focus of attention is competition from other stimuli that can cause a shift in focus. This is because one maintains focus on a particular thought by inhibiting all potential distractions and not by strengthening that central mental activity. Directed attention fatigue occurs when a particular part of the brain’s global inhibitory system is overworked due to the suppression of increasing numbers of stimuli. This temporary condition is not a clinical illness or a personality disorder. It is rather a temporary fatigue of the inhibitory mechanisms in the brain.

Essentially, you're working your brain more when you're trying to focus on something boring while on the other hand the job wouldn't be as hard if you were doing something interesting. Because of that mental exertion, you may feel sleepier and more exhausted. 

Signs of Directed Attention Fatigue include temporarily feeling unusually distractible, impatient, forgetful, or cranky when there is no associated illness. In more severe forms, it can lead to bad judgment, apathy, or accidents, and can contribute to increased stress levels.