.. it may sounds strange but the symptoms you described are the same that make hypnosis work. When i was sitting in an auditorium some professor make me feel excatly like you feel. Its mainly a mental issue in combination with a relaxed mental health (slow pulse rate, comfort feeling, etc).
I read a lot about hypnosis and all i can say is that it is your mind that causes feelings like that.
That implies that you can change it if you want by training yourself (autosuggestion).
The easiest way to prevent that feeling would be keeping your heart rate up somehow.
(you said you don't want to take coffein or ephedrine so you have to find other ways. Black tea for example (ok, it has coffein in it too but it's meeker to your belly). Autosuggestion can do that too, it would be the "master way")
If i find a good side about hypnosis in english i will post it ...
