OMG JAL learn a thing once in a while. Benadryl is ALSO classified as an antihistimine that has the side effect of drowsiness. Doxylamine is the active ingredient in UNISOM - a sleep aid. It is an antihistimine that also causes sleepiness. That does not mean it will have the same effect on every human on the planet, we are all different. Benadryl doesn't make some people sleepy either. Sudafed is a decongestant that more commonly makes people nervous/jittery as a side effect, though it does make SOME people sleepy, that is less common and there are no commercial preparations that advertise sudafed as helping anyone sleep. It's the antihistimine in Nyquil that provides the sleepy effect, always has been and still continues. The active ingredient in Zquil is diphenhydramine which is benadryl, but like I said that medicine is very similar to doxylamine anyway, they're in the same classification.