Probably some of the best advice in this thread. I'm surprised by all the moralising and severe responses from other folks. (exercising with a hangover is very dangerous? Really? Please explain how)
Hangovers cause severe dehydration. It's an extreme case, but a British soldier died last year from a combination of a severe hangover and punitive exercise. Just fell over on the running track and didn't get back up.
Ok, but there's a big difference between "severe hangover and punitive exercise" and "ow I've got a headache, I'll still do at least a mile this morning".
I can imagine that exercising with a hangover would suck, but I don't know how it's dangerous....