The way I see it... If we look hard at the creation stories, we are going to see that those who built Nirn had nothing else but magic to make it from. "Physical reality" of Tamriel is pure magic - but limited (thanks to Lorkhan's nature). This is why alchemy works. So, everything is magic, including light. Light - sunlight for sure, at least - is the least bound/limited form of "material" magic because Magnus has escaped the material prison.
I've never heard of magic being less effective at night or inside caves.
Gameplay!=lore, but one of the most awesome aspects of TESII:Daggerfall chargen was the ability to choose how your character's magic abilities depend on the environment. You could get "light-powered magic" or "darkness-powered" - which translated to more spellpoints in preferred environment or to only being able to cast spells in the preferred environment. Spell absorption was also a choice that could be checked against the environment - you could choose spell absorption "in light", "in darkness" or "general".
Artificial lighting, in general, did not really seem to count for "in light". //the actual implementation of this feature is a bit glitchy anyway...//
Granted, Daggerfall was released before the major rules of Aurbis were established, so technically "darkness-powered magic" would make little sense, but with enough misinterpretation, you could write a passable lore-friendly "scientific" explanation of your character′s backstory, even if it includes diminished spellpoints "in light" yet spell absorption being active "in light" only as well. Yeah that's my character =)
But in light of such customising being unavailable in later games, I don't know if it would make sense for any character but a specifically genetically engineered Altmeri experiment...