It's crazy what the human brain will remember and for how long, and what we can't remember for even 24 hours (or less). Some will say it depends on importance to the person, but there's some things that don't add up with that. After almost 40 years, I can still quote you the phone numbers of my two childhood best friends, my cousin next door and my grandparent's next door in the other direction's phone numbers. Now why do I need to remember any of those, when none of those #'s have been in use for years, and not by anyone I'd need to call for over 35 years. Can rattle off my wife's SSN and driver's license info as fast as my own, and state hwy #'s like recalling multiplication tables. People that aren't from a rural area (and even some that are) are baffled by that, you get used to calling a road by it's "name", well none of those roads have road signs stating that, so if you're giving someone directions and they're not using GPS, they need those road numbers to see those signs. Getting pretty rare that someone doesn't say "I'll punch it into my GPS" but I'd sold stuff to some folks my age and older who still do it by brain function. 

Ask me what I ate yesterday or what short little task I accomplished, and I might have to think about it, or I might rattle it right off... and it be what I ate or what I did TWO days ago, honestly thinking it was yesterday. That part continually gets more likely to be wrong by the year it seems, and I'm not even 50. I don't know whether to be concerned that it might be a little early for that or not!

That being said, looking back at how my grandfather's behavior would shift late in the day/early evening when he was getting into the mid-stages of alzheimers, I feel like I at least partially understand it. I, still today, remember my childhood/early teen years vividly. There's things I can remember that aren't even remarkable that I can playback like watching a recording. I don't mean one or two things, I mean an entire catalog of videos. There's moments I can recall like that in my adult life, and even though much more recent, they are much fewer and more prone to being a little foggy or missing details. As my grandfather would near the evening, it's like the calendar would roll back 50 years. He thought I was my Dad, and would yell at me from his wheelchair to "Go on now and get those mules back in the barn before supper!" I don't know when the last mule left, but the first tractor didn't come until sometime in the 50's, a Super C so likely 52/53 or so. My Dad graduated high school in '54 and went into the Air Force that fall and was around to use it before he left, but apparently grandpa's reality was taking him a little further back than that. People offer a lot of concern that it must be awful for the person, but the jury is still out for me. There's times I saw him happy as a lark, living in that world, I can only imagine what his reality was, was he seeing my grandmother for the first time all over again? Was he enjoying an afternoon fishing on the creek bank? I don't know if science will ever get us inside of what happens, but I choose to believe that in those moments, they are really in that time, and not torn between two worlds in their mind.