Most snails can't actually see images and instead see how light/dark something is. Your snails likely come out because they see light. Though honestly snails don't really care whether its night or day, they just sorta do their thing.
Some of mine used to come out during the day too, but after I put puffers in the tank I would only see them at night. I think they either learned they'd be harassed if they came out during the day (unlikely) or the "day snails" all got eaten and only the night owls survived.