good listening/bad listening

Do you ever listen to your system and say WOW that is great? It really sounds excellent.

Then, other days you say, ehh why does it sound like that...I don't remember it soundin like that?

So, my question is, is this physcoacoustics or.....???????
There is no "bad listening" from my system. I always think to myself: wow, that's fantastic sound, that's real high end. Well, I don't bother "thinking" that all the time: it's just there as a matter of course.
For listening during conversations or vacuuming or other chores I have a second little system in the same room which is astonishingly good. But for serious listening the high end system only..... and no regrets!!
There are definitely physiological aspects that affect our listening. The most obvious is when we have a cold (as Shadome pointed out) and our ears are stopped up. Imagine now, what if your ears were stopped up, but you didn't really have a cold and didn't recognize it? Your system would sound muffled just as if you had a cold.

I know there are things that I do that have a dramatic effect on my listening. If I haven't had enough sleep things don't sound very good. If I haven't excercised (I have no idea why this has an effect, but it's pretty profound for me, particularly at the high frequency). After flying my hearing is not as good (and I wear ear plugs to reduce this effect).
My system sounds better at night when the lights are just right. It doesn't sound as good when bright sunlight is coming thru the windows. My wife and I both agree on this phenomenom. We know it's not "true" but it's real to us. This is why I suspect that many tweaks and upgrades don't really improve the sound but we think they do and therefore may be worth it. Humans are weird. Audiophiles are weirder still.
Guys saying that it might have something to do with your emotional state at that particular moment. Due to the higher stress level it might just not be enough to overcome our desire to distance ourself from difficult situation.

Night time might also have something to do with it.
I am not sure if it has anything to do with the power but I also enjoy it more offten during late hours.
I think the power grid is quieter at night as is other interfence--such as road noise. Road noise you may not hear directly but the vibrations through the road--particularly if you are not too far from a highway is pretty amazing. So night listening being superior is something I would guess most of us experience--I do.