The characters' total unfamiliarity with darkness, even on a world without night, seems a bit overstated: don't they have thunderclouds on their planet? Caves? Mining? Fortresses, that'd need to be built without windows? You'd think that lamps would have been invented for these purposes, if no other.
It's stated that they do have these things and people that experience it, like the archaeologist, are uncomfortable with darkness but stand up fairly well. The main characters likewise being prepared and knowing the exact length the darkness will last (9-14 hours depending on latitude) do MUCH better than the general population. There's a big difference between experiencing a couple of minutes or hours of darkness knowing exactly when it will end and where to go to find light and being plunged into darkness unexpectedly and without knowing how long it will last. Not to mention that one effect often ignored is the presence of STARS which suddenly makes people contemplate their true insignificance in the universe. Spend five minutes thinking about how small WE really are, now imagine you've NEVER contemplated that before and you have NO KNOWLEDGE of a universe beyond the solar system and are suddenly seeing MILLIONS of stars.
There is one scene that takes place during a rain storm. One character finds it dreary and depressing. The other character, who is still traumatized from experiencing darkness for 15 minutes, has to fight off a panic attack. Also, everyone keeps nightlights called "god lights" in their homes for when they have to sleep and such.
From the novel: they have electric lighting, but the power grid is utterly incapable of powering every single light being turned on at once.