"I will both lie down and sleep in peace;
for You alone, O LORD, make me lie down in safety." (Psalm 4:8)
It is an act of trust: I am reminded when I go to sleep that the world is in God's hands, not mine. The world will get along very well even though I am not awake to try to control things. At the appropriate time, my eyes will open and I will receive the gift of wakefulness once again.
"I lie down and sleep;I wake again, for the LORD sustains me." (Psalm 3:5)
-- John Ortberg in “The Life You've Always Wanted”