Albert Barnes Commentary Psalms 91:5

"Thou shalt not be afraid for the terror by night, Nor for the arrow that flieth by day;" — Psalms 91:5 (ASV)

You shall not be afraid for the terror by night - That which usually causes alarm at night—a sudden attack; an unexpected incursion of enemies; sudden disease coming on by night; or the pestilence which seems to love night, and to walk in darkness. Any one of these things seems to be aggravated by night and darkness; and for this reason, we most dread them then. We cannot see their approach; we cannot measure their outlines; we do not know the extent of the danger, or what may be the calamity.

Nor for the arrow that flies by day - Whether shot from the bow of God—as pestilence and disease—or from the hand of man in battle. The idea is, that he who trusts in God will be calm. Compare the notes at Psalm 56:3.