I have a theory I want to record, just for the sake of recording clinical observations.
 
There is a neural reflex within the sensory fibers of the Fifth Cranial Nerve, and the sympathetic and parasympathetic supply to the nose.  When you have a cold or other cause of nasal congestion, and you lie on your side to try and sleep, there is a sympathetic reflex which causes vasoconstriction in the superior nostril, opening up the nostril by inducing vasoconstriction.  When you roll over, the formerly superior nostril becomes congested, and the newly superior one experiences vasoconstriction, again allowing you to breathe.  This happens in the absence of significant mucus (or with mucus), but is not simply the result of gravity-driven mucus flow. It happens in a dry, allergic nostril, just as it does in a wet, secretory nostril.  This is an “N” of one, but I am sure it is true.