Your friend is in town and hits you up for a night out of drinking. You go, of course, but wake up the next day with a massive hangover. Why?
We know alcohol dehydrates us. Not only that, it acts as a diuretic, which means more frequent trips to the bathroom. That and a lack of water could be the cause of a nasty hangover.
Drinking too much of something with a high sugar content will trigger insulin to be released. The blood sugar fluctuation may be a reason for that weak, shaky feeling.
Finally, when you hit the sheets to sleep after a night out, you aren't getting much of it. Alcohol prevents you from reaching the REM stages of sleep that's needed.