I've taken unisom for HG as well (it worked MUCH better for me than prescription meds ever did --plus I liked being able to adjust the dosage/timing as needed) but don't know about how it would affect labor. It may be you'd have prodromal labor anyway even with out the unisom? If this has been going on for some time, though, I'd talk to your care provider about ways to get more sleep (perhaps herbal remedies or medication). You don't want to be utterly exhausted and sleep deprived before your labor ever gets going.
Best wishes,
Best wishes,