The last time I did W in 2001, I slept at about 4,000 ft. elevation the two nights previous, with a few hikes up higher during the day. Did the r.t. hike in 15 hours, October 12, btw. I'd always do well when I was bagging the high peaks, I'd spend the 2 day weekends up high, then rush down back to work for the week, and head up for another weekend, back right to where I was as far as acclimation, it seemed.
One memorable trip, we camped two nights at about Lamarck Col, 12,000' elevation, and I spent the tues-fri week at sea level. The next weekend, I dashed back up to bag Willie. The two with me had come from sea level without any prior acclimation and were pretty sick. I was great, could have bagged Tyndall if I had the inclination.
Though when I bagged Shasta, without any acclimation for weeks, I got a standard headache. More anecdotal reminiscence, but so true.
Physiologically, you are at altitude and your red blood count goes up accordingly. A few hours or, by me, even a work week back down at sea level and you retain some of your higher red blood cell count. Or so it worked for me, by me!