Actually, the aliens see in the dark as well as your own men do, it's just that the aliens are cleverer than your men, here's what they do (shown from the side):
Key
@ - Alien
% - Your soldier
{ - Your line of sight
> - Alien's movement
_ - Ground surface
* - Plasma beam
It starts off like this:
@_{_______%
Then the alien moves into your line of sight, sees you and shoots at you:
__
>{>@
*****%
Then the alien moves just out of your line of sight:
@
<{<_______%
Here the turn ends, with your being shot at but never seeing the alien.
(It took me ages to do this
)