I figure the power line noise is one possibility, though those with power filtering should notice less difference, right?
I've also found that at night there are less distractions. I prefer to listen in the dark AND close my eyes because I've found that when my brain is processing input from my eyes, I get less involved in the music. I've found when I close my eyes during the day, I still don't notice all the nuances I do at night. It also might be that during the day, there's almost always something else in the back of my mind that comes next. At night, I'm getting to enjoy music with nothing else between me and going to sleep. I also usually have a glass of wine at night too, where as during the day, I don't.
It could be some or all of those things. For me, I notice a difference and have a preference for nighttime listening.