What significance can you see in the fact that Grendel attacks at nignt in Beowulf?
There are two general answers to this. The simplest and most literal answer is, in this time of primitive technology, a night attack is most threatening. The humans have no lights except fire light, and that flickers and smokes, threatening to burn them even as it lights their homes. On the emotional/symbolic level, Grendel is a monster, a creature of darkness, and something of a nightmare. Grendel comes from the dark as fears come from the subconscious.