system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I get negative thoughts the longer I stay up?

The reason I ask is that I'm generally a content person. I enjoy things in my life and have a good time as much as I can.

However, it seems that the longer I stay up (late into the AM) I have a tendency to lean towards negative thoughts (usually when I'm alone). Does anyone know why or what causes this?
gpt
Steve peters explains this pretty well in his book on psychology, describing the chimp model of the brain. You can roughly think of the brain as being split into 3 parts, the computer, the chimp and the human.

The human is roughly equated to being the frontal cortex, this area deals with conscious thoughts, logic and facts and is directly under your control. The chimp is roughly equated to being the limbic system, it is the area of your brain that deals with emotions and basic survival instincts. It is the area of the brain that kicks in in fight or flight reactions and is not under your control. Finally the computer is the area of the brain that deals with memories and automatic thinking. Both the human and the chimp refer back to the computer in order to make pre determined decisions about how to react or deal with a situation based upon previous experiences.

Due to the fact that the 'human' requires active thought and decision making, this area of the brain is slower to react than the 'chimp' portion. It also becomes less effective if your are tired, hungry or otherwise not on top form. In these circumstances the 'chimp' will take over causing you to make decisions based entirely on emotion rather than logic. This is the case when you are in bed trying to sleep, gradually the 'human' is shutting down leaving the 'chimp' free roam to take over. This is why often as you are falling asleep you will find your thoughts become much more disorganised and irrational, and why problems you think about might seem terrible and unmanageable, whereas in the morning, when your human wakes up and properly analyses the situation, it all just seems silly. 

Edit: grammar.