system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How does being tired lead to irritableness?

I'm in my finals week in law school this week, and I haven't had a proper sleep for a long time.


I'm usually extremely patient, calm, happy and fun. However, at the moment I have no patience for any BS and the slightest thing can set me off.


What biological reasoning is there behind this? ELI5 please :)
gpt
One of the more "evolutionary modern" functions of the brain is restraint. We can suppress primary reactions like irritation and anger because we can reason why that would benefit us. However, it takes some energy to use this function of the brain and when energy is low or your brain is tired from not getting too much sleep, this just doesn't work that well anymore. 

There's only so much energy to go around and when it's scarce it's appointed to more vital functions. In short because being alive is more important than not being an a**hole ;)