I think this is because your inner voice is doing this at late night:

"Dude, what the hell are you doing at this ungodly hour?"

"I am trying to get _____ done"

"Oh okay"

So, you are continuously conscious and aware. Naturally you are more productive. I create the same phenomenon during the day too by using a simple tool that I wrote for this exact reason: Smacklet http://www.smacklet.com

( shameless plug, but I mean it )