"But it sounds better at night...."

High end audio is so much about power supply. The more expensive the piece of gear the more effort, cost and weight goes into regulating and isolating the power. Note most high end components are two box with one dedicated to power. No major problem in your power is required to have power conditioning or regeneration. Most, by far, but not all systems sound is improved by these devices.
 

I have observed quieter at night for at least thirty years. It is real. It is most likely to be due to a quieter power grid, at home and locally, the Bliss stuff above and micro vibrations. 
 

I am going to order a seismometer this year and verify what I have suspected for a long time, that it is seismically far less active at night… particularly late Sunday night… the quietest of times. I suspect the traffic, probably within a mile is a contributing factor. I am looking forward to seeing if this verifies what I hear (as a contributing factor).

I would imagine 

Another consideration I was thinking about is at night, I personally have a chance to listen for longer periods of time so there could be a factor of your ears settling in / adjusting during a longer listening session. 

But I do concur, I do feel there's a difference listening at night even if it's just a perception.

alcohol.

alcohol.

I was just about to type that I’m far more likely to have a beer at night, and looked up and saw your reply! It’s the best $5 improvement you can make. 😁

I also wonder if a darker room can contribute....the system usually sounds better when I close my eyes, as it helps me focus on the sound rather than the birds flying by.

Less ambient noise, thanks to less traffic in the neighborhood and up above.