As a 175 pound male, I have drank that much and taken the dose of Benadryl that you mentioned on a number of occasions. Not all the time or anything, but it's something I've done from time to time. Certainly, it never resulted in "getting a good sleep" though. Just basically passing out either in bed or in my recliner and waking up feeling like shit the next day. So, I'm quite surprised that you felt like you got a good sleep. However, mixing that much alcohol with Benadryl certainly can't be good for your physical health. I usually drink 10-12 units myself a night, but when I go beyond my usual amount I typically get a hangover. My "chill out" dose booze wise is usually a tall boy of high alcohol beer (a nice imperial stout or imperial ipa or if I'm on a budget a steel reserve) along with a couple shots of 100 proof vodka or a couple 16 ounce cans of 6-8% craft beer and a couple shots of this interesting vodka I drink that has a vanilla flavor to it. Even with my usual amount, I have never felt like booze really gave me a better sleep. Just a "buzz" that I enjoyed and perhaps helped me to get to sleep earlier as sometimes without alcohol I'll stay up late as fuck (e.g. staying up until 5 AM). I don't know why. I'm self employed, so I likely wouldn't stay up so late otherwise but since I am I still don't really like staying up till 5 sleep schedule wise even though I'm self-employed and can set my own schedule. Overall, I wouldn't recommend drinking 20 units a night to anyone. In fact, according to rational doctors (e.g. not the CDC's information of "two standard drinks a day for males and one for women" which I feel is nothing other than government brainwashing and state-sponsored propaganda a.k.a. fake news in my opinion), drinking more than around a bottle of standard ABV wine 10-12% or around a six pack of 4.2-5% of beer a night is potentially dangerous as far as liver health though the effects vary for everyone.
However, not everyone experiences liver damage. Some who drink more than this amount, even by a lot are fine. However, others who exceed this amount experience long-term deterioration in liver function that can even prove fatal in some cases. According to statistics, around 20% of people who drink amounts that are large enough to cause this damage experience problems with their liver. Still, drinking 20 units a night, on a typical night is definitely too much and could cause serious problems. As far as I know, there is no particular risk with regard to combining alcohol and diphenhydramine in terms of immediate effects. Although, adding another substance to the mix certainly increases the strain on your body to remove it. In the immediate sense, I've suffered no ill effects from this. Though, mixing them both regularly would increase the potential for issues. Also, I suggest taking some nights off from alcohol if you also drink every night. This will likely help to prevent damage from it and also lessen your chances of physical dependence if you are a "drink every night" type of person like I am. As much as it seems to "suck" to not drink at night, as a daily drinker, I find that 90% of it is psyching myself out. And the actual experience of not drinking one or even two nights is not a big deal, I just build it up in my mind as a "big deal". So, after having seen this, in a way I can take it or leave it to be honest. Drinking at night is a mere preference for me and not a necessity. If you drink every night, I'd also really recommend going without for a night to see how that is. At least, take a night off every now and then.
curious how they interact, since i dont want CNS depression x)
