So's my roommate.
I'm an ISFP. Over the years, I have actually learned that I need less introvert time than I thought I did, but it is still vital to my well-being. The key is balancing it with being social. If I get too much of the former without the latter it eventually stops being beneficial and I just lose track of my free time and go back to work more stressed.
THIS is the best article on introverts I've ever read. I think @ad infinitum was the one who introduced me to it.
Also, a quote I saw the other day:
"Socializing is as exhausting as giving blood. People assume we loners are misanthropes just sitting thinking, ‘Oh, people are such a bunch of assholes,’ but it’s really not like that. We just have a smaller tolerance for what it takes to be with others. It means having to perform. I get so tired of communicating."
Anneli Rufus