First I think we need to recognize that any talk of Signal-to-Noise ratio is beside the point. There is noise in the environment and in the source, but we are dealing with digital data and the manipulations done by JRiver are 64bit. Further, we are talking about a situation that is inherently not bit-perfect. As soon as volume leveling is turned on, JRiver is manipulating levels (or volume, if you want to use a vaguer term) ALL THE TIME. Using Volume Leveling and Dynamic Range Compression is not about making SNR the best it can be, it's about making perceived volume sound appropriate to the ears. Just as room correction and EQ are about altering the sound, and they also do it by manipulating the levels. Since JRiver does the manipulation as cleanly as possible, let's embrace it.
BryanC raised the question of only boosting the quiet portions and not peaks, and mwillems was correct about the way recordings are mastered to a peak value.
The trick is that while there may be only one mathematical peak in the track that reaches 0dBFS, to the ear the loudest 10-20% are going to sound like peaks. An explosion is an explosion; they're loud. The fact that this one is .001% quieter than the loudest one doesn't make it sound less of a peak. But due to the way I think Night mode is behaving, that explosion will get boosted anyway.
I've created these charts. (input level is horizontal, output vertical) The first one is the way I think Night Mode is currently working.
As mwillems observed, it's going to boost everything except the mathematical peak. This is not what I would consider friendly behavior, as it actually makes a lot of explosions louder. I've verified this conjecture with JRiver's analyzer, so I'm fairly confident this is what it's doing.
The 2nd and third charts are friendlier ways to do it. Peaks are cut, and quiet parts are boosted. These are the ways consumer audio equipment with 'Night Mode' functions work, like Audionut's Yamaha.
The third chart uses two thresholds, so that peaks are cut, quiet parts are boosted, and an attempt is made to preserve the dynamics of the middle portion.
Blgentry's question about thresholds was right on the nose. I think the answer looks like the first chart, regardless of the actual numbers. But we'd all be better off if it looked like the 2nd or 3rd charts.
Such an approach would better preserve average volume, while still compressing dynamic range. Making "everything" (99.9%) louder, as in the first chart, is counter-intuitive to what "night mode" should be.
So again, we need someone from JRiver to weigh in on what it's actually doing, and address if that is the best way to go. Sorry for the long post!