In reply to icehockeyhair:
I don't disagree about the cervical(neck) hypothesis,but I have a damaged neck w a plate across C6-C7,and get stiff and painful when I belay for a long time outdoors.Yet I do not get headaches overnight with it.
I would still drink plenty on getting home,forget about salt as there is loads in any commercial food that you eat,and take a pint of water to bed to drink in the night.I find the latter to be really useful(apart from sometimes drinking too much before going to bed and then more in the night,and wondering why I need to pee).