Quote from Calacbolg
The light would be calculated on the fly. To put it simply, a certain block can't NEED to have updated sunlight and be unloaded at the same time. If it's unloaded, then no effects of unupdated sunlight can apply.
Actually I think what schmidtbochum was saying is the current system of top-down light would not work. This is true, at least for infinite height. If height limits were left in place then the code could simply grab blocks at specified coordinates in chunks above it to figure out light without actually having to load the whole chunk. If said chunk(s) is(are) already loaded then this wouldn't be an issue.