Incentivized Sleep-dep-o-naut-ery

Random question: in all of the make-something/elimination reality TV shows that I know of (like Project Runway and Face Off), the contestants are always only given a certain amount of time during the day in which to work on something. Even during a multi-day project, they can only work 8-12 hours and then have to be taken back to their cages. Why is that? It seems like, if you want drama and ill-advised creative efforts, letting them stay up for 48 hours with an IV caffeine drip working feverishly on a project while yelling at their hallucinations would be a no-brainer. Could it be a liability thing? Or maybe they just don't want to pay for the film crews? Seems weird to me.