Blinded by the lights

The other day, J and I were driving from Seamus McDaniel's to Ted Drewes. It was after dark, and I was looking out the passenger side window. Most everything was dark, and the lights that were on -- front porch lights and such -- were pretty dim. Then we passed by a small side street, and there was a car sitting a little ways down the street with its headlights on.

I couldn't make myself look away fast enough; before I knew it, I had looked right into the extremely bright headlights. And for just a few seconds, maybe five or ten, I had a splitting headache.

So my question is this: why does that happen? I know that your pupils shrink as the amount of light increases, but is that what makes your head hurt? Or is it the optic nerve?

Well, whatever the answer, it is not fun. So make sure you look away faster than I did!