Why is it bad to use a nasal spray with oxymetazoline hydrochloride nightly?

Tachyphylaxis: Basically your nose will become desentized to the effect of the oxymetazoline. If you use beyond the recommend time (3-5 days), your nose will upregulate receptors for it and require more and more for the same effect. There are better options for nightly nasal stuffiness and simple in-office procedures can be a permanent solution for you - see your local ent.