Maybe it's just getting older, but I've noticed feeling 'unacclimatized' more significantly after weather changes recently. Hot or cold days seem to surprise my

Maybe it's just getting older, but I've noticed feeling 'unacclimatized' more significantly after weather changes recently. Hot or cold days seem to surprise my body more than they used to. We had a crazy heat wave in Seattle this year. I wonder if that could be related physiologically somehow. What are the physiological systems involved in adapting to high temperatures? How long does this adaptive effect last?