Why did God put Abram to sleep in the covenant ceremony?
God caused a deep sleep to fall on Abram so that he would have nothing to do with the covenant transaction. The ceremony was entirely God's doing, picturing an unconditional covenant that depended solely on God's own faithfulness, not on Abram's performance or participation.
Les explains that God put Abram to sleep just as He had done with Adam long before, calling God a master anesthesiologist. The purpose was so Abram could have nothing to do with the transaction. This shows the covenant was not a mutual agreement but God's own unconditional action, with Abram passive while God alone passed through. Afterward, God begins to give genuine prophecy, telling Abraham that his seed would be strangers in Egypt, afflicted four hundred years, then judged and brought out with great substance, and would return to Canaan in the fourth generation.
And when the sun was going down, a deep sleep fell upon Abram; and, lo, an horror of great darkness fell upon him.