Are humans more adapted to "light mode" or "dark mode"?
A question that requires quite a lot of guts to ask on this site :) Nonetheless, and risking sparking a debate, there are a few arguments that spring to (my!) mind that can support the notion that we thrive better in 'day mode' (i.e., photopic conditions).

To start with a controversial assumption, humans are diurnal animals, meaning we are probably, but arguably, best adapted to photopic (a lot of light) conditions.
A safer and less philosophical way to approach your question is by looking at the physiology and anatomy of the photosensitive organ of humans, i.e., the retina. The photosensitive cells in the retina are the rods and cones. Photopic conditions favor cone receptors that mediate the perception of color. Scotopic (little light) conditions favor rod activity, which are much more sensitive to photons, but operate on a gray scale only. The highest density of photoreceptors is found in the macular region, which is stacked with cones and confers high-acuity color vision. The periphery of the retina contains mostly rods, which mediate low-visual acuity only. Since highest densities of photoreceptors are situated at the most important spot located at approximately 0 degrees, i.e., our point of focus, and since these are mainly cones, we apparently are best adapted to photopic conditions Kolb, 2012).
An evolutionary approach would be to start with the fact that (most) humans are trichromats (barred folks with some sort of color blindness), meaning we synthesize our color palette using 3 cone receptors sensitive to red (long wavelength), green (intermediate) and blue (short). Humans are thought to have evolved from apes. Those apes are thought to have been dichromats, which have only a long/intermediate cone and a blue cone.  It has been put forward that the splitting of the short/intermediate cone in our ape ancestors to a separate red/green cone was favorable because we could better distinguish ripe from unripe fruits. Since cones operate in the light, we apparently were selected for cone activity and thus photopic conditions (Bompas et al, 2013).

Literature
- Bompas et al., Iperception (2013); 4(2): 84–94
- Kolb, Webvision - The Organization of the Retina and Visual System (2012), Moran Eye Center
Further reading
- Why does a light object appear lighter in your peripheral vision when it's dark?