Unknown. No studies done with regards to melatonin and dementia.
There are studies. No studies specifically for afib and melatonin I am aware of, but many studies for melatonin in ad.
Yes. Melatonin has been studied recently for a natural treament for those with ad for both cognition as well as restoring better sleep. A starting dose on 2 mg 2 hour before sleep maybe effective. There should be no interaction between meds for afib and melatonin.
Yes. But, it can increase the sedation associated with tranquilizers and anti-psychotics that some alzheimer's patients take and it can potentiate the heart rate slowing effect of verapamil that some atrial fibrillation patients take.
Prob. The question is whether we can figure out what is the cause of insomnia in a person with dementia---at times it can be caused by sleep architecture breakdown from the illness itself; at times from poor sleep hygiene; at times from meds or alcohol; at times from a sleep disorder such as apnea; can be from depression;; so important to first try to get to the cause...