They are anticholinergic, similar to jimson weed and scopolamine.
At lower doses, doxylamine is a significant hypnotic, at higher doses anticholinergics cause hallucinations. I have read that high doses do disrupt sleep, people tend to wake up evrey so often, look around, pass out, only to wake up again a few minutes later. Some people, especially children, might become euphoric on higher doses. Most people just get sleepy and irritable, but don't sleep well
From my experience the hallucinations are 100% integrated. You can't really tell the difference between them and reality. Antichilinergics produce true hallucinations, and are the only hallucinogens that I know of that are capable of this. This might SOUND fascinating, but it's not. It just like every day life. Every day life that is really anxious, sleepy and jumpy, with weird stuff that goes on around you - but, you don't really even realize it's weird. Pretty much the only thing you notice is the anxiety. Even today I don't know how much of what I saw while using diphenhydramine/diphenhydranate was actually a hallucination. The stuff lacks novelty entirely. Hallucinations are tend to be people, animals, bugs, and other everyday objects. Smokers tend to smoke phantom cigarettes.
They can be dangerous. Aside from their tendency to fuck up your heart, you have no way to know if you are hallucinating or not, because of this a person might interact with the hallucination in ways that might cause injury. There really is no safe way to trip on anticholinergics. They are pretty unpredictable. No matter how experienced you think that you might be, nothing can prepare you for them as you cannot distinguish where the trip begins and ends.
Aside from anxiety and being easily startled, thee drugs are completely devoid of all emotional context. This is a good thing, really. If one were to be paranoid it could get very bad. But it doesn't make for a very fun or useful experience. Many people who are new to hallucinogens mistakenly believe the visuals are the primary aspect of the psychedelic process, and read that anticholinergics are unique in that they produce true hallucinations. However, there is no psychic context in these visions. They are as uninteresting as everyday life - no matter how bizarre.
When used/abused similar to other hypnotics, doxylamine can be kind of fun as a body fuck.Taking 50-100mg with alcohol is fairly euphoric and mentally stimulating, while physically relaxing. I don't know how dangerous this combo is, but common sense says to limit your alcohol consumption. I drank 1 shot Everclear (equivalent to about 2 shots 80-proof) and was thoroughly fucked with 100mg doxylamine.
(ETA: The cocktail was drank over a 15-30 minute period. I wouldn't slam a couple shots of vodka with doxylamine in your system and expect the same result since drinking over time will result in lower BAC. Note that I was thoroughly plastered. Usually 1 shot Everclear will get me pretty buzzed or mildly drunk. That evening I was pretty much falling over drunk. If you're going to be stupid like this, at least be smart about it and approach with caution. Stay safe.)
But as for using anticholinercs as a hallucinogen, I can appreciate your intrigue. But save yourself the trouble. These chemicals are just not worth it. In fact the only OTC that is, IMO, is DXM.