This thread is just......ridiculous? I cannot believe anyone would recommend taking prescription medication and alcohol at the same time. Those little stickers prohibiting alcohol consumption are not just on your pill bottles to provide additional decoration. It is beyond irresponsible to do this yourself, and much worse to suggest to a college student that it is a desirable way to travel.
I rarely drink while flying. It just does not seem to be a wise idea. I am a huge believer in living better through chemicals, but I would not use any of them irresponsibly. If I want to sleep I take a prescription medication manufactured for that purpose. I do not guzzle Nyquil. I do not recommend that anyone use medication any differently than allowed for in the dosage instructions. Many of those medications depress your respiration rate, and when mixed with alcohol it means further depression of your respiration rate so sometimes you just stop breathing. Even if you have been successful with this behavior in the past does not mean you are immune to killing yourself by mixing sleeping medication or cold medication and alcohol.
OP, drinking Dayquil to stay awake during finals is not the reason you did not get the sniffles. These is no prophylactic for the common cold, and I suspect that it did not do that much to improve your concentration. Likely it was just a placebo effect. One hopes, for the sake of your future career, that you have not shared this information on My Space.
How about watching the movie, listening to your iPod, reading a good book and being sure you do not arrive at your destination in a coma!