The Midnight Effect - Who-How?

You have high end equipment designed in a way to make it seemingly impervious to power line fluctuations. You add expensive conditioners and/or power line regenerators just to be safe.

You sit and listen to your system for a few hours and everything sounds great. Then, from nowhere, like someone flicked a switch…. the sound opens up… becomes more natural, more focused… the soundstage suddenly blooms and becomes more dimensional, more depth and more space around instruments. WTF just happened? The only clue is the clock on the wall and the empty wine flagon next to your chair.

I’m long past questioning whether the phenomenon is real. To what extent it exists depends on certain variables, but it exists. But how? I live in the boondocks, there’s no industry or commerce that suddenly shuts down at 23:00 every night. 
Do others experience this? Do you have an explanation? Perhaps even some empirical data?

Is it just the booze?