SO apparently if you add enough instant coffee to make it taste like a "strong" cup of coffee... its gonna contain way more caffiene than you expected. Didn't realize it until my hands wouldn't stop jittering.
It actually tastes pretty good but yeah. Don't be like me. Looks like I'm definitely staying up until midnight.
Caffeine withdrawal for me is vascular headaches completely unaffected by any painkillers, and nausea. I can swing "grumpy and tired" by halving my dose every couple days.
I probably didn't relate those to nausea because that's just high dose caffeine for me