In just about every one of these “ways you know you are getting older” threads, someone says they can’t drink as much alcohol as they used to without unpleasant aftereffects. Usually, I assumed this to mean being more susceptible to hangovers, but one thing I find as I get older is that if I have alcohol too soon before going to bed I’ll likely not sleep well. Does anyone else find this to be true? FTR I’m 48, and it’s only recently I’ve noticed this particular phenomenon.
I have only anecdotal evidence to offer, but I’ve definitely found it to be true. In fact, if I have too much to drink too close to bedtime, I can pretty much count on waking up around 3 a.m. Other folks I’ve talked to who are in their 40’s have noticed a similar problem.
Your experience sounds a lot like what happened to me last night. And it wasn’t a huge amount of alcohol, but more than I would usually have.
I’ll have to be more careful in the future. One good thing about being older is that you do pay more attention to consequences. 
I’m in my mid 40s and find that about a 6 pack makes me sleep much better.
I’m 27 and always, always, always wake up at about 3-5 am and stay up for at least 2 hours after a night of drinking heavily. It’s been this way since I was about 18, or ever since I starting drinking excessively.
So I guess my point is that it doesn’t only happen to older people.
I don’t have a cite (sorry!), but I seem to remember reading that you wake up because your BAL has dropped, and the body wants more alcohol.