Could dehydration be causing my headaches?

[CookieMonster]

I have just been reading an old thread about the amount of water we are supposed to drink each day and wondering if this is what is causing my headaches.
I have a headache for roughly half of my waking hours - most of the time it is just a nagging ache (always in the same place in my head) but sometimes it gets bad enough to have to take aspirin. It rarely keeps me awake or wakes me up at night, maybe once a month.
I do use a computer terminal all day but I have had my eyes tested and am OK and surely I would have headaches all the time if it was my eyes that were the problem.
People have always told me that I don't drink enough fluids but I just find it impossible to keep drinking all the time. When I have tried it in the past it just makes me feel bloated and uncomfortable and I need to keep going to the loo. In a normal day I drink about 4 cups of very weak milky tea and the occasional glass of diluted squash i.e. one glass some days not others.
Does anybody know whether dehydration could cause such a problem or should I be looking at other parts of my lifestyle?

stuartc--I've also noticed a corrolation between decaf coffee (instant in particular) & headaches. I thought maybe it was a coincidence. . . What on earth is in that stuff? Sorry you're still feeling grotty, CM.

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I don't have any problem with normal coffee - only the decaffeinated stuff.
About a year after I'd worked out what was causing the headaches, there was a newspaper article which suggested that headaches and decaf often went together. It also said that there were two different processes for decaffeination and one process was more of a headache problem than the other.