Look into the research behind melatonin use. I’m not a doctor and certainly long term use of diphenhydramine is associated with neurological problems in old age, but I’m not sure melatonin should be used as a simple hypnotic as you are suggesting. It’s natural but so is testosterone. Hormones may not be good to tinker with. I say that as a long time user of melatonin. At the very least you may want to stick with lower dosages- nothing over 1mg which is about as low as is easy to find.