Sleep – Hebrew:  yishenah – sleep, to repeat, to change, to understand, divine revelation.
Song of Solomon 5:2 “I sleep but my heart is awake, it is the voice of my beloved that knocks.”
The word Yishenah comes from two possible root words.  The first is Yishen which simply means to sleep.  The second is often overlooked by Christians as it does not seem to fit the context, yet for the ancient sages it fits very well.  The second root word is “Shanah” which is the same root word for Shannah in Rosh Hashannah.  Shanah means to do over again and again; it is also means year, and change and new beginnings.  In its most primitive form it expresses the idea of divine revelation, peace and the power of God.
The word is spelled Shin (completeness, wholeness), Nun (moving forward) and Hei (the presence of God).  Esoterically the word sleep has the idea of God completing us, making us whole so we can move forward with His presence during the day.  This is how the ancient Jewish sages viewed sleep, it was a time for God to complete us and make us whole so that we could move forward in the new day with His presence.
If God has limited man to just 120 years on earth, why did He create us to spend one-third or forty of those years in an unconscious state?  The ancient sages teach that it is only our physical natural bodies that are unconscious; the spiritual part remains awake and allows God to share His heart with ours while we are sleep.  However, God does not live in time, he lives in the past, present and future so if He shares His heart with us during our sleep, we know what He knows including the future.  That is why prophets received their Word from God in dreams.  That is why we forget our dreams or they take on symbolic meanings because some knowledge we receive in this union of hearts would be too painful for us.
“I sleep but my heart is awake” is a very sweet romantic phrase which carries very deep spiritual insight.  The lover may be asleep but her heart is awake, waiting for that knock at the door of her heart.  Her lover is off in a palace, she is in her simple home, yet she believes even separated by many miles or even in the same bed that while they are asleep the physical barriers are knocked down and her beloved can knock on the door of her heart and whether in the same room or the same bed, their hearts can still embrace while they are asleep.
When we are asleep, the physical part of us, that part filled with the cares of this world, our lust, our fleshly desires are out for the count and our hearts stand open and vulnerable to the influence of spiritual forces.  Fill your wide awake heart with a movie of zombies and vampires and the enemy will come knocking at the door of your heart.  Fill your heart with the Word of God and praise before going off to the land of nod and it is Jesus who comes knocking at the door of your heart.  Which heart does yours embrace the enemies (nightmares) or the loving Father’s heart (sweet dreams)?