Works very well to reduce the severity of the hangover. Putting back at least a half litre to a litre is advisable. You might need towake up early to take a leak, but that's the whole point - to help flush your body of alcohol and keep hydrated. Usually if you've been drinking heavily, you're alredy pretty dehydrated, so much of the water will just be absorbed. Most people with a healthy bladder won't be getting up to pee all night.
Popping a vitamin as well can't hurt, as alcohol does deplete certain vitamins.
