To encourage players to take on night time missions instead of skipping them, how about giving unique bonuses on completion? This could be a specific medal, increased experience points or an extra cash bonus. It does seem inconsistent to have a huge spike in difficulty on night time maps with no compensation. And also unrealistic to be able to play 99% of your missions in daytime by waiting for time to pass.
(NB Thanks to all for making this fantastic game - im really starting to feel the immersion for the first time with the latest update)