Kat, you are right, the daytime nap test is more about other sleep disorders (narcolepsy and idiopathic hypersomnia particularly), than about just OSA. It is also to contrast the type of sleep you have during the day and the night. I wonder in hindsight why the sleep doc didn't explain the need for a MLST to me before my test. He may have just felt, as most sleep specialists do, that it is simply part of a thorough sleep evaluation.
Unfortunately it did give cause for concern in my case, and I was very bothered by being told I have to stay for an extra nap without being given an explanation otehr than "you went right into REM." It wasn't till later that I realized I had triggered alarm bells for narcolepsy. They could have given some informative but non-committal explanation, "You have an unusual pattern of sleep, so we want to do an extra test to see if it happens every time or just once." I guess some people actually like knowing no more than they have to, but nowadays I think most of us want to know what we are being tested for!
Sleeping with a curvaceous blonde autoPAP (Resmed autoset). Hope springs eternal.....