Know that Yahweh has set apart the holy for himself! - Psalms 3 & 4

Both of these Psalms talk of God relieving his set apart people from their enemies, and both Psalms make very fitting and applicable evening prayers before going to bed, especially during times for Christians dealing with difficult people who are opposing God’s purposes.

Now, the setting of Psalm 3 is apparently during the attempted coup d'état of David’s son Absalom. The events surrounding this period are in roughly 2 Samuel 15-17, which sheds light on David’s feelings and the pressure facing him. David knows from God’s promise to establish his dynasty and have him rest with his fathers (2 Samuel 7) that this coup won’t succeed, right? This is why his confidence is in Yahweh to protect him and why he can boldly ask him to “Arise and rescue.” Yahweh is faithful to his promises. He is who he is (hence the name). This is why David can sleep secure, even if uncountable enemies surround him and seem to have the upper hand (as Absalom seemed to have had).