Very inflexible rules.  I was told if I didn't sleep long enough in the first part they may not be able to do the second part, and then I might have to come back another night for the second part.  My Dr. prescribed a split study...if possible.   They shouldn't have discouraged you.  It's their job to sit there all night monitoring you. 
I'd complain to my Dr and ask if there is a different sleep study company to recommend.  I did mine at a suburban Hospital.