Tis the season to be jolly. And when you've finished being jolly, tis the season to wake up with veisalgia, more popularly known as a hangover.
The punishment begins when alcohol reaches your brain and tells your pituitary glands to stop producing a hormone known as vasopressin. It's vasopressin that regulates your body's water retention, and without it your kidneys begin to drain your body. With all those trips to the bathroom, you'll also be flushing away essential salts and electrolytes, leading to fatigue and nausea.
Alcohol also inhibits the production of glucose, the energy source for your brain. When you finish drinking, glucose production goes into overdrive. For most of us, that's right when we plan to sleep. Your hyperexcited brain won't fully shut down, you endure a restless night and wake up foggy.