One thing I know is pretty well-researched about light, is that our brains produce dopamine in response to optic-nerve signals hitting our
https://en.wikipedia.org/wiki/Suprachiasmatic_nucleus. This response is what causes circadian entrainment, but it also just has the same effect as any other dopaminergic agonism in the brain: increasing motivational arousal. This is why light-therapy lamps warn you not to use them for more than 30 minutes at a time: the excess dopamine produced by your SCN will put you into a state of mania.
Up to a point, people work better with more dopamine in their brains. (Many people have subclinical/undiagnosed ADHD, and work especially better with more dopamine in their brains.) One way to make people's brains produce more dopamine is to make them stare directly into bright light sources. Therefore, light-mode is very likely to be observed as "more productive" for anyone who's not already saturating their dopamine receptors by other means (drugs et al.)
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Fun tangent that I like to bring up, because few people seem to have heard of it:
This dopamine agonism in the SCN also increases muscle tone in muscles with nerve connections afferent to the SCN, e.g. the orbital muscles of the eye (chemical "feedback" through the optic nerve, essentially.)
The current hypothesis for the development of myopia (nearsightedness) is that the eye's orbital muscles must receive a certain amount of dopamine during the development/growth of the skull to keep the eye pressed into the right shape relative to the ocular orbit. In people who don't get enough bright-light exposure to consistently trigger dopamine release from the SCN, their orbital muscles remain slack, allowing their eye to "relax" wider into the orbit, leading to the retina growing into position further away from the cornea, and thus leading to myopia.
This means that children don't get nearsighted from doing "close work" all day; but they do become nearsighted from staying inside all day, where they aren't getting enough SCN stimulation from the lower indoor light levels. The likely "fix" for the epidemic of nearsightedness is to get kids to do even sedentary activities outside. (And for people who live in places where it's cloudy/raining most of the year, to get indoor lighting equivalent in light output to a light-therapy lamp.)