Hello my dad has PD and dementia and he doesn't sleep well at night . My mom is the main caretaker and she's burned out by getting up all night cause he makes noise and always confused. She tried from melatonin to other natural sleep aids to prescriptions and nothing is working.
Ironically, when my mother with Alzheimer's started getting worse with getting up several times at night for these phantom urges to pee, a different doctor prescribed a different pill, Seroquel, and we do the same thing--it's the lowest dose, which we then cut in half. It's just enough to take the edge off. She usually sleeps 6 hours straight through. Anything stronger and it's zombie time.
Both of these medications were prescribed "off label" and are not actually intended for the use these doctors prescribed them. Seroquel, in fact, has very stern label warnings about not being suitable for persons with dementia. Yet I have watched for side effects like a hawk and have not noted any. At one time a few years ago they tried another one on her (could probably look it up if you are curious) and it didn't work at all so I was grateful that the Seroquel did work.
Again, 1/2 of a 10 mg pill isn't very much medicine at all so maybe if you do want to try it you could ask the doctor about Elavil (amitriptyline hcl) which is basically for depression, which a lot of PD patients have anyway.
As for OTC, let me say upfront that I prefer natural aids or OTC to pharmaceuticals and did try them first. To anyone with a dementia patient using melatonin, be watchful for reactions to it. I read somewhere that it can make some people violent. Google "can melatonin cause aggression" and you will find lots of articles on it. I don't know about the whole Nyquil thing... sounds promising. Some say that Calms Forte, a homeopathic OTC medicine, works well but I never tried it on my Dad (who had PD dementia); did try it on my mother (Alzheimer's) and it didn't work.
Regarding sleep issues; most of the Parkinson's meds will work as you indicated. Also, another Lewy Body problem is REM Sleep Behavior Disorder. You can Google it or find info in the previously mentioned books. As a 'last resort' antipsychotic meds such as Clonazepam or Seroquel could be tried. Bear in mind that whatever has been tried and not worked may be an indication of high drug sensitivity. That is, when a patient takes a normal dose, the body sees it as an overdose, sometimes a huge overdose.
You didn't mention the type of doctor your dad has been seeing. For PD, it’s usually a movement disorder specialist. Typically, they know very little about dealing with dementia so it’s best to find a Lewy-savvy neurologist. Most of the time you have to ask about their Lewy qualifications.
Lewy Savvy
Bear in mind that as PD changes and progresses, different medications are used, so it is important to let the MD know what has changed to he can modify the drug regimen.