There are 2 related parts to this question:
1) Haman is annoyed with Mordechai and discusses with Zeresh, his wife and his wise advisers what to do. In Esther 5:14, they tell him to make a tall tree, and in the morning tell the king about his plans.
We see that Haman didn't quite listen to his wife or his wise men to go in the morning and instead went at night. Was he so delirious about his plan that he forgoes the advise of his own wise men, and furthermore dares to disturb the king's nighttime rest?
(Granted, the king was awake, but Haman didn't know that, and seems that he wanted to awaken the king in the middle of the night).