Remember, especially if it is dark, the perception of the sound field and the imaging will seem better. Since there are less visual cues competing for your attention, your ears will be able to accept the 'illusion' better and perceive more.
Also, I agree, do NOT underestimate electrical interference, noises, ingress, etc... These are massive factors in some areas and if you are not plugging your equipment into AC filters/cleaners etc... (which do not current limit the amps) you are probably getting less noise at night than during the day.
Also, at night, we tend to be calmer, and the din of the day has gone down and I for one believe that different times of the day induce subtle yet very potent psychological states or tendencies which in general are conducive to headphone listening (unless the music is abrasive).
Neil