From a biological view, alcohol is a toxin that the body scrambles to get rid of. The liver has to work overtime to break it down, while the brain tries to adapt to the chemical chaos. If you drink regularly, the brain tries to fight back by turning down its natural brakes and turning up its gas pedals to stay awake. This is why heavy drinkers feel jittery and anxious when they stop: their brain is running on overdrive without the alcohol to hold it back.