Ferret is absolutely correct visit the sleep center prior to your testing, also check out the Physician that will be interpreting your results, make sure that they are Board certified and have experience in treating patients with narcolepsy.
Please relax there is nothing you can do to change the outcome of the test other than to not stress over it.
Actually you do not have to fall asleep within 8 minutes on the MSLT. What we look for is if you fall asleep and if so is REM sleep present.
It is important that you watch carefully what medications you are taking as there are a great deal of REM suppressants.
Do not worry if you do not get a diagnosis from this test, remember these folks are only getting a glimpse of your life in 20 hours you may need to repeat it again and if so then you will, just dont stop looking for answers.
I would suggest keeping a sleep diary about two weeks prior to your test date and give it to the Physician on your appointment.
I wish you all the best