I recently stumbled upon a fascinating Wikipedia page that just might describe a phenomena that I bet just about every one of us CW ops have experienced at times. You've just finished off a marathon CW effort such as a contest or Field Day and finally get a chance to lay down, close your eyes, and try to get some sleep in a nice quiet room. Almost too quiet. You're drifting to sleep...and then you hear it. Strains of CW, just on the edge of your hearing. You can almost make out some meaning, but it's not quite coherent. It's just your mind playing tricks on you.
The article which I found describes a condition known as Pareidolia, as Wikipedia sums up quite succinctly:
Pareidolia (pronounced /pærɪˈdoʊliə/ pa-ri-DOE-lee-ə) is a psychological phenomenon involving a vague and random stimulus (often an image or sound) being perceived as significant. Common examples include seeing images of animals or faces in clouds, the man in the moon, and hearing hidden messages on records played in reverse.
Long story short, our brains are supremely attuned to pattern matching, probably as a survival mechanism. At times, it is likely that it leads us to perceive meaningful patterns where in reality there are none. An extreme example would be those people who see images of Jesus in their grilled cheese sandwich or that nutty Reverse Speech guy, but we've all experienced minor versions of it at some times in our lives.
It seems that there must be some aspect of immersing yourself in CW that makes you even more susceptible to the phenomena for some time afterward. In my experience, sometimes it's taken an hour or two to shake it from my head. Even when I haven't been recently working CW, there are times where some random squeaks or beeps perk up my ears and get me in the CW copying mind frame. It's a funny and peculiar thing, to be sure.