I've done a few tests on Night mode readability vs day mode, and when it's best to use them
I put it in a small chart:
There are 3 environmental settings, and 3 illumination settings.
The environmental settings are:
1- Bright (outside)
2- Office (~300-500nits)
3- Night/dark (bedroom at night)
The illumination settings are:
1- Back light (either light mounted behind, or inside the display)
2- Front light (light either mounted on top of the display, or as an outside light shining on the screen)
3- Natural light (environmental light)
There are 4 possible results:
1- Night = night mode; dark screen white text
2- Normal = day mode; white screen black text
3- Both, could be seen as 'either'
4- X = not a viable way to read
In my tests I found that in outside
(bright) light, it's MUCH better for the eyes to use Night mode in ALL 3 illumination settings.
The outside light (sun) is overpowering most back or front settings, and reading a white page in a bright sun is hurting the eyes.
In all
Office light scenarios, it was a matter of preference, with a front light having a slight edge on normal mode (as night mode blocks out too much light from front lit screens, and may make the text harder to read in night mode).
In all
dark room (night) environments,
The natural light was not an option, as one couldn't read an ereader with some illumination (and at night ambient light is close to zero).
The front light, same as the office setting, preference set to normal mode, as night mode bleeds out the light, and makes text harder to read.
The back light, was a matter of preference. While both were very close to one another in terms of readability, the night mode had a slight edge for me due to there being less light bleed, possibly bothering a partner sleeping next to you when reading in bed.
Those are my personal preferences.