Yes - perspiration from your body, also your breath if you have your mouth inside the bivy (don't).
I have found that, generally, I lose about 2 lbs of weight per night. This leaves my body in the form of urine, sweat, and vapor from my lungs.
I guess that about a pound of that is urine, so that leaves about a pound from lung and skin vapor. The vapor from the skin migrates outward until it reaches an area where it is cold enough to condense, certainly the bivy inside surface and also the outer layers of insulation in the sleeping bag.