Well, Perhaps LV-426 is so far away from it's sun or the sun is so dim that there is little different between day and night. The day is a lot like twilight and the night is pitch black.

Roaches

We don't know the true planet of origin of the Aliens. Also, they have not been studied under laboratory conditions. Thus it is impossible to know their exact sensitivity to sunlight. If they somehow originated on a planet as far from its primary as Pluto is from the Sun, normal daylight for them would be no more that very bright starlight to us. Should they have come from a planet of a red dwarf star, they would, again, probably react differently to the light from Zeta-2 Reticuli than do the humans. Finally, any photophobia they display might not be to the visible portion of the spectrum. If Newt is right, they might be avoiding infrared (IR) or ultraviolet (UV) light which is invisible to us, but might affect them.

Night for people...

Aliens probably attack at night not because of sunlight, which is almost irrelevant inside colony. But people living in it use *day* and *night*, in the *night* they mostly sleep. So aliens attack, when there is quiet and sleeping people are much easier to catch.

Nocturnal

The veracity of this explanation depends on how one interprets the Aliens Vs Predator novels as cannon. I personally, do, as they offer insights into the nature of the Xenomorphs. They are a nocturnal species, which doesn't like the heat of day. They tend to sleep either in their hives, or in another cool, dark place during the day. At night, when it's cooler, they become active.

Quiet!

When a bunch of people are up and around doing stuff, it can be pretty loud. On the other hand, when a bunch of people are all sleeping, it's usually pretty quiet. The aliens just wait for it to get quiet in there, and then they go have their way with the place. And if you take Comment #34401 into consideration, that'd make it nighttime, wouldn't it?