I can't remember what lead me to this train of thought, by just recently I was thinking about Valerica and the basic nature of vampires.
(Here's your warning. If you haven't done the Dawnguard questline; thar be spoilers ahead me matey)
So I was thinking about Valerica being trapped in the Soul Cairn, and the fact vampires "live" on drinking blood. Vampires require blood, and she has been trapped in a castle for who no how long in a dimension where the only other being of flesh that exists besides herself was the dragon that is supposed to kill her. It can be assumed she never got any blood. That means she has gone without any blood...for a long time.
She likely became a vampire some time before the Second Era, as Molag Bal managed to stop by to personally rape her. Judging from Serana's reaction to the notion of a Cyrodiilic Empire, the two have been isolated for somewhere between 4207 to 4337 years. I can't go 24 hours without feeling the affects of bloodlust! Serana may have gotten off easier than Valerica, as she was in some kind of hibernation state, and perhaps that little blood ritual that we did to release her had some affect on her. Valerica on the other hand had no such luxury.
Valerica in fact seemed perfectly fine for going without any blood, or food, or water for over 4000 years. And she only had like 5 books! Why is she still sane?