@barts — good points. I wouldn’t disagree but I would counter slightly with the observation that a constant seems to be the hour of the day at which the effect “switches on”. I haven’t done anything remotely scientific to verify this, but it seems to happen a little after midnight, give or take a half hour or so. So regardless of whether you start a listening session at let’s say 7pm, versus starting a listening session at say 10pm, the switch-on time is the same. So things like relaxation, state of mind, biorhythms etc, would seem to be secondary causes to whatever it is that is happening.
Anyway, it’s an interesting enough phenomenon and one that adds another element of spice to the hobby.