12 hours seems like a long time for the first time bound.
However, it certainly is something that could be done.
This guy did it for 3 days with no major ill effects.
http://theoriginalinstitute.com/cell21Main.htm
Also, mental patients in the past have certainly been bound in a straitjacket for longer than 12 hours.
If I were you, I would start off with an hour or two or and work my way up. Personally, I think I'd love it. I have spent a few hours, but never 12 .