Eva Head is the condition most people experiance after watching/ reading too much (or in some rare cases any) of Evangelion. In this case Evangelion encompases a manga series, a 26 episode anime, and two films: "Death and Rebirth" and "The End of Eva." Eva Head has been described as a hardening of the liquid around ones brain(or atleats it feels like it) accompanied by loss of sense of reality, sensation of floating, confusion and sharpness of vision. This is because there is way too much information to process and Director Anno's true message is so disturbing that the brain wont allow it all to sink in. Generally this is experienced after watching multiple episodes of the series. Eva fans ideally consider episodes 1-20, plus the diectors cuts of episodes 21-24, plus The End of Eva to be the entire story, this will take 8-13 hours, and I gaurentee you that if you watch it all in one sitting you will get Eva Head. Eva Head can last anywhere from 20 minutes to 3 months.
Eva Head can also mean the mind set one must be in to watch Eva and atempt to understand what is happening. It is simlar to idea of putting on one's "thinking cap"
1. 1st Guy: Jeff just finished Eva.
2nd Guy: Oh no! He's gonna get Eva Head. We probably shouldn't talk to him for a few months.
2. 1st Guy: We got 13 hours to kill, wanna do a marathon?
2nd Guy: Alright, time to put Eva Head.