Alcohol typically affects the brain in two phases. When you first start drinking, it can make you "feel good," lower stress or make you feel more sociable. If you continue drinking, it can have sedative effects which is why you may feel sleepy shortly after the happy feelings.
Scientists using scans of the brain have discovered that alcohol releases endorphins - the "pleasure hormones." These endorphins then bind to opiate receptors in the brain. As a result, the brain is rewarded for consuming alcohol with pleasurable feelings.
Contrary to what some people believe, alcohol starts to affect your body from your first sip. That means that even the smallest amount of alcohol can take a toll on your body's systems. As soon as you drink more than your body can process, and your blood alcohol concentration increases, you feel the effects of being drunk. This distributes alcohol throughout your body, numbing your nerve endings, and slowing down brain function.