Frankly this question was asked by my Chevrutah today.
We see that many things in the Torah are caused or moved by sleeping and dreaming - Yaakov dreaming of the Ladder, Yossef having fights with his brothers over his dreams, Pharaoh having dreams and starting the Egyptian Exile etc.
However, in the Purim story, it is exactly the opposite - Achashverosh having sudden insomnia and being unable to sleep and have dreams and things started to roll.
Why would that be?