on muscle memory

So, I've been up since 2:15 continuing my work on my project. The door opens, and I'm all like, I wonder who that could be at four in the morning (nevermind that I had breakfast at 3, and am having lunch now, which is about when this happened). I hop up to check the door, as I often do. There's Eric coming in, but I must not have recognized him, because all of a sudden I notice my hands are up in fists in a normal defensive posture.

Needless to say, Eric was a touch surprised.