What is the penlaty for attacking (or for that matter the bonuses as a defender) at night?
Is arty more or less effective at night?
And to what extent are dawn or dusk periods impacted by such effects?
I think the bonus for defending at night would be Armour bumping into infantry defending. The Armour nearly always comes of worst due to its limited visibility, and its inability to use its optics, and long range fire power.
Of course the bonuses for infantry attacking at night are huge.
It allows your attacking infantry to sneak up on the enemy positions, and mitigate any longer range bonus they had for defending from cover, while the attacker crosses open terrain to reach them, and by the time they are spotted its to late to bring heavy arty to bare as well.
Also mutual support from other defending units can not be brought to bare, if they are defending in depth, or dispersed to mitigate artillery damage, where as the attacker can use tight formations to bring max fire power onto a single enemy unit as they advance.
Best counter for this is one of your own infantry Battalions in reserve to counter attack the attacker, in a tight formation preferably where it hurts most i.e. in the flank or even the rear.