It's based on the sleep tracking data my fitness watch gives me, but also just based on how I rested I feel the next day, since I know there's limitations to the data quality a watch can give you.

I have noticed the early waking thing with alcohol, but for me that's never really an issue after a drink or two - only at higher consumption levels do I really have that problem (along with the bad dreams/nightmares that many have after drinking).

One other thing I've realized while experimenting with this is that I tend to be sleep better when I'm cooler - so anything that would raise my body temperature has a huge negative impact on sleep quality. This could be large amounts of alcohol, or it could be lots of sugar in the evenings, or a salty/starchy meal late in the day, etc. This seems to be a more important variable than anything else I've observed.