I'll throw in a personal experience with caffeine. I was feeling really bad (anxiety-attack-ish, according to Google) whenever I sat down to work on my thesis,

I'll throw in a personal experience with caffeine. I was feeling really bad (anxiety-attack-ish, according to Google) whenever I sat down to work on my thesis, a few years back. I'd use nice coffee to bribe myself to work on the thing. One day I went to work on the thesis after going to the gym and left my heart-rate monitor on. I noticed that this bad feeling coincided with a seated heart rate of 110 bpm, long enough after exercise that it should have been around 68. Investigation and experimentation ensued. Short version: caffeine + hard thinking about something scary launched me into a real physical anxiety attack; just switching to green tea allowed me to continue my work ritual without the panic.