At the labs I have worked in the procedure is that you have to have a "normal" PSG before you can have an MSLT. So since this is your first sleep study if they find sleep apnea they would cancel the MSLT, get you a CPAP and let you try that for a while. After you are compliant on CPAP for a while and if you are still having trouble then you can have a PSG on CPAP with an MSLT following. I would guess that the MSLT is overkill. Did you fill out an epworth scale in the office, a few of the questions can indicate narcolepsy which may be while they ordered the MSLT. Who knows. I can never figure out what Dr's are up to.