What was so special about the night that Achashveirosh couldn't sleep that it was referred to in the Megila (6:1) as "that night", i.e.

בלילה ההוא נדדה שנת המלך

It could have said something more typical like

ויהי בלילה אחד

The language of "on that night" seems to be unusual. Was there really anything special about that night or the day that preceded it that we must know they are connected?