Darts, shields, civil wars—sounds like a violent little poem to us. And violent it certainly is— metaphorically that is. We suppose that's better than real violence, but it's still no fun. The speaker is in the middle of a war—with Despair, who won't stop shooting darts at him, and also with himself. Yep, in a way he's his own worst enemy. There's a civil war going on inside him—a battle between all of his emotions, thoughts, and desires. And there's really nothing civil about it. If there was, he probably wouldn't be so eager to go to sleep and get away from it all.
Lines 5-6: The speaker is being attacked by the "fierce darts" of Despair, and he needs Sleep to protect him. This is personification, clearly. What about the metaphors though? Well, Despair isn't really throwing darts, so this is a metaphor for the way the speaker feels despair and sadness are like being attacked by stinging darts. Ouch. The shield of proof, and the act of shielding, are metaphors for the ways in which sleep can protect one from these things.
Line 7: The speaker wants Sleep to put a stop to the civil wars going on inside of him. Yep, Sleep is still being personified, and these civil wars are definitely a metaphor for the speaker's emotional turmoil and inner conflicts.
Line 11: The speaker offers his "rosy garland and weary head" as a sacrifice, but it's pretty self-serving. By "taking" these things, a metaphor for Sleep's ability to ease the speaker's suffering, the speaker is asking for some emotional protection, in a way. It's like saying, "take this brain of mine is feeling so good and make it better, protect it from itself."