Now when the sun was going down, a deep sleep fell upon Abram; and behold, horror and great darkness fell upon him. Genesis 15:12
Yikes, usually a deep sleep is a cause for celebration. A deep sleep seems to become more difficult to attain in my later years, but if this is going to happen I'm not sure I want to keep trying. Reading back in the story though we find doubt, uncertainty, fear the Lord will not do just as He said. Yes, Abram wanted a confirmation sign after the Lord had given His word.
The word of God is the very best assurance because of who He is. We doubt Him at our peril. The Lord said that He is coming back to take us to the home He went to prepare for us, and that word is all we need. Did Abram's descendants get 400 years of slavery because Abram would not believe without a sign? We hope not for Abram's sake, but that doubt did bring a prophecy of the years of toil they would face in Egypt. However, the Lord also comes back with that prophecy of a large Israel, one that has not yet been seen upon the Earth. Perhaps Abram was able to get some better sleep with the thought, "I messed up again, but God is good!"
Read the word of God, and have some sleep without doubt,Bucky