system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do you feel nauseous or sick after staying up too long?
gpt
During sleep, our bodies replenish a lot of key substances which are necessary for the normal function. This is regulated by the 'circadian clock' which is just a fancy word for our bodies' sleep/wake clock. 

When there's sleep deprivation, we have an absence of these key substances being replenished, and that in itself can cause some of these symptoms like nausea/headaches. 

Alongside this, it creates stress on the body, and that activates our 'fight vs. flight' response. One of the effects of this response being activated is also having feelings of sickness/nausea. 

It's a lot more complicated than that because there are so many things that work together, but this is the very basic gist of it.