It is remarkable how little sleep actually occurs during a sleep study. Of course, the first couple of hours are spent with no CPAP so that they can see how frequently you stop breathing while sleeping. (Hint: in my case, a lot.) This is followed by connecting the CPAP to a mask and gradually ramping up the pressure. Since my current machine is set to 16, it took a lot of ramping before getting to an effective level.
But I think I got about two hours of actual good sleep. And they should have all of the data they need now as I try to get a new CPAP set to the proper pressure.