good listening/bad listening

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).

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.