I've been polyphasic for a short time (sleeping less than 4 hours per 24 hour period) and I've been sleep deprived for longer, having one sleep at night of five hours or less a day for a while, and they feel like two completely different things.

Not to say that polyphasic is healthy or anything, but despite the slightly weird feeling, I feel much more functional when polyphasic than when sleep deprived.