This scripture starts off with a crying out of to God. David is in distress, but he is reminded to be still when he prays upon is head. In the end of the scripture David does not seem as distressed. Rather he knows he can rest in peace at night, because this is how God commanded it to be. Not only that, but he knows God will keep him safe. God will keep us safe through the night, so that we can rest peacefully even though we have had a stressful day.