Echoing what others have said, power-line issues could explain what you’re hearing.
In my case, my house had been powered by a rooftop PV system (just panels, no batteries at the time). During the day, my stereo (kinda mid-audiophile, costing a total of maybe $25K at the time) sounded pretty good, albeit a little too bright & Class D-ish for my taste. But at night, it generally sounded better. A lot better, in terms of soundstage, imaging, and transparency. And recordings that I found hard to listen to during the day lost much of their D-type graininess and harshness after the sun went down. Huh?
It didn’t occur to me at the time that at night, the house switched from solar power to the grid. In case you’re not aware, the inverters integrated into most rooftop-PV systems produce far more noise than a typical electric utility. Was that a contributing factor?
Then, a few years ago, I sunk 8 grand into an Audioquest Niagara power-conditioner + power cord upgrade. Using a consumer-grade line-noise meter I confirmed that the Niagara reduced my line noise by at least one order of magnitude. (Thanks, Garth Powell & Mikey Fremer!)
And you know what? Not only did the Niagara make a night-and-day improvement to my SQ overall, but now there was now no difference between "night and day" sonics (sorry!) I found that I could even shut off the entire PV system & run off the grid without a change in sonics.
Confirmation bias? Nah. I didn’t expect this to happen, so there was no belief to confirm. It was only afterwards, after consulting with some solar-power & line-conditioning engineers & contractors, did I finally figure out what was going on.
YMMV, but the differences I heard were dramatic, and were later corroborated by objective noise measurements. Nothing else changed in my system at the time, but it had never -- never -- before produced such a palpable, three-dimensional presentation or so convincintly reproduced the acoustic environment of a live performance. For a few weeks, I just couldn’t stop listening to my favorite, most familiar recordings.
So whenever someone asks about time-of-day variations in the sound quality of an audiophile-grade system, I always think first about time-dependent, cyclical factors that can affect AC line noise.
What I’ve learned: If I had the choice between spending $10K on either an amp upgrade or a mains upgrade, I might well choose the latter.
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Oh, and to address the OP’s original question (yeah, it’s not all about me!), dramatic changes can occur in the quality of a utility’s delivered power as a function of time of day.
E.g., many utilities adhere to a daily network-switching schedule that accommodates the technical limitations of alternate power sources (such as solar plants v. coal-burning), or that routes power differently depending upon average time-of-day-related demands. If the OP thinks that the change in sound quality happens at about the same time every non-holiday weeknight, it might be worth a call to his or her utility to ask whether any change in topology or other delivery details occurs at a regularly scheduled time.
