Histamine is a brain chemical. Some people say it's an important one; others call it quack science. It's interesting to me because so many things fit. I've read that the high histamine person tends toward obsessive, phobic fear while the low histamine person is more likely to be paranoid. I also read that the high histamine person can have an unusually still energy on the outside while on the inside their brain is going fast, fast, fast.
I imagine histamine is one of many neurochemicals that can have an effect on a person's functioning -- though of course that doesn't mean that a particular behavior is always associated with a particular imbalance. In my case, histamine does have an effect. Some foods speed up my mind in a bad way. Food that's past its peak will set my mind racing. I think about the food I ate during my years at the housing co-op: nearly spoiled produce bought at $2.00 a (large) box. Not so good!
The effects of caffeine? Caffeine seems to calm my mind. Antihistamines and caffeine can have similar effects on me. Both can be somnolent. And both, oddly, can be conducive to work... at least at extreme moments. There was a point where I took stronger medicine, but I told the doctor the antihistamines worked better.
On histamine, my thoughts are like molecules in a solid. They tend to move around in a tightly packed space. I can get to the point where it's hard to complete a sentence or action; those fast thoughts are repeating or going in circles instead of moving forward. On caffeine, they reach escape velocity. There's forward motion.