Do you struggle to find peace amidst the unfairnesses and injustices of life? Imagine being able to sleep peacefully without your mind racing and tumbling over a thousand and one things. Imagine having a joy that is rooted, not in circumstances, but in somewhere that nothing can touch.
That's what David points us to in this Psalm - not in a theoretical way, but like watching a live video feed: He starts out crying "Answer me" and finishes falling asleep in peace - because... well you'll have to listen to find out.