since it really doesn't get dark anymore anyway, why disable missions at dusk or dawn? really lame.
Once upon a time... We had little sheep that frolicked the lands of Aces High... Tankers adored them as loving pets and company during long spawn camping... Then one day, a mysterious force came to befuddle our once loving sheep... Whenever the sun goes down, our Sheep used to turned into fierce monsters that attacked missions and campers alike upon spawning... Finally, the great demon killer Skuzzy came and destroyed the evil-doers with his mighty golden Ban-hammer...
This was done to protect you and your score...
This is also why we no longer have sheep...