Another theory - perhaps you are overtired. This might be ordinary fatigue, rather than listening fatigue.
The same sort of thing happens to me, but usually in slightly different circumstances. At times I find it impossible to sit down to read because I just fall asleep. I think that, when overtired, we depend on a whole array of stimuli to keep us awake - especially visual ones: computer screens and TV sets for example. But when you immerse yourself in stimuli that engage you in a less beligerent way, your body just wants to shut down. It's not so much that the music or reading is relaxing (in fact, reading for work is rarely relaxing, and "critical" listening can be hard work), but that the music or the book doesn't keep you awake the same way that watching TV, or reading AC threads online, or "keeping busy" might.
Headaches may be consistent with this, as a physical reaction to the fact that you are now trying to stay awake, rather than allowing whatever hormonal operations do the job for you. And when you get up and leave the stereo, and "get busy" again, do you really feel refreshed? Or do you just feel awake again? Perhaps you've just managed to find some more effective stimulation to keep you awake.
My suggestion is to turn the stereo off for a couple of nights, and get some serious sleep.
Chad
