system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what’s happening in your body when you’re hungover that makes you feel so crappy?
gpt
There is still a lot of debate regarding the causes of hangovers and it is challenging to study.

There are a few factors that are thought to be the main contributors to feeling so crappy.

1. Your body has an inflammatory response to the alcohol, inflammatory responses don't feel good
2. Alcohol alters the levels of different neurotransmitters in your brain and this takes time to return to normal
3. Alcohol damages your mitochondria (part of your cells that make energy).
4. The metabolites of alcohol (chemicals that your body breaks alcohol down into), particularly acetaldehyde, are toxic.  Acetaldehyde is usually broken down really quickly so probably has minimal impact for most people, unless you are genetically less able to break it down (its build up in the body causes 'Asian Flush' in some East Asian people for example)

Edit:

I should have included something about dehydration up here. Dehydration definitely contributes to how you feel in the morning but it is not solely responsible for the actual alcohol hangover.

This is the reason hangovers can still occur with appropriate hydration and why they don't go away simply by rehydrating.

It's generally considered that the effects of dehydration often occur concurrently but should be considered separately to the effects of the alcohol.