I have tried different types of coffee, all causing sleepiness. Except the opposite affect with iced coffee which helps with wakefulness. What causes for this?
Disclaimer: I'm not a physician, so here are only guesses based on my experience.
Besides coffeine, there are other substances to be taken into account, plus some non-chemical effects:
Sugar and milk by themselves can have relaxing effects. It is a well known fact that sugar can induce sleepiness, and hot milk is often drunk before going to bed. I've started drinking black coffee (no sugar) after lunch because of this.
Milk and coffee together are particularly hard to process by the stomach. This can bring a sort of after-lunch drowsiness.
There may be a psychological effect related to the assumption of coffee, or any other drink that is associated with relax and calm in your mind.
Warmth can have a relaxing effect by itself