I'll be interested to hear what folks say about this as I suffer from the same thing - mostly when going from sitting to standing. For me it doesn't have to be the next day, just a few hours after a hard workout will do it for me.

d

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"Life is fragile - we are all just a slip or a car crash away from being a very different person."

Going from only personal experience and no real medical background or science behind it- just alot of sweating- and having my father fainting and take a trip to the ER (at 55yo) after back to back cycling days (not crazy for him, but first 2 riding days of the year seemed to do a number on him)
I'm going to chalk it all up to electrolytes.
Dehydration and electrolyte imbalance- somehow tying into decreased blood pressure. Dad took 2L of fluids in his IV before he got his color back. Thats pretty much all they said- sweat out too much, didnt replenish fully, and it kicked his ass.
Maybe you think you're getting enough, but are underestimating?

are you tall?

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

I'll continue my hyjack to keep this up front for a while.
I don't know what most people consider tall - I'm 6'1" and I sweat an incredible amount, borderline ridiculous, puddles under my trainer after only an hour. It does not happen when I wake up in the morning, but rather if I'm sitting at my desk at work and I stand up quickly.

d

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"Life is fragile - we are all just a slip or a car crash away from being a very different person."

This happens to me after really hard workouts, races, or when I have just gone overboard in training. I think it's a pretty good sign that your body is worn out. If you are fit then you probably have pretty low blood pressure and fatigue and other factors can contribute to what you are describing.
Chad

sounds like a blood pressure or hydration issue to me. I would see my doc to rule out anything ominous.

Yes, this has happened to me. I have low blood pressure, and when combined with either low hydration or just laying down and getting up too quickly, I can become very faint and even pass out. I think the doctor called it Vaso Vagal or something similar to that. I was suppose to take a "tilt test", which is some kind of test they do in the hospital where they try to rule out other things. Since he was pretty sure it was Vaso Vagals, he just told me to take my time getting up. With the low blood pressure, there is not enough pressure to get the blood to the brain fast enough.

I will add, though, that if I take extra salt (electrolytes) during the day, it doesn't happen either. Freaks out the wife, though. I drink a ton of V8 and try to work the others (calcium, magnesium, etc) into my diet.

If it keeps happening, a trip to the doctor is not a bad idea.

that happens more in taller people

not sure it has anything to do with your sweating
happens to me, 6'3", and i dont sweat an unusual amount

http://en.wikipedia.org/...Postural_hypotension

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

Many endurance athletes have low blood pressure. I have as well and I am frequently light headed - not just after hard workouts. Also runs in families.

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I ride a Cervelo...get over it....

triyourbest, it is likely a BP problem. Jackmott might've given you a good link - google "orthostatic hypotension"

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maybe she's born with it, maybe it's chlorine
If you're injured and need some sympathy, PM me and I'm very happy to write back.
disclaimer: PhD not MD