There isn't any way I know of to make this perfect; but you can emulate the effect by adding the item 'Sight Range Bonus' ability with a negative value to units when they enter the region, and remove it when they leave.
This will give units 'night' vision while they're in that area, if the value is roughly -800 to -1000. You may need to remove it during night, because it could remove all of the vision from a unit.