Causes

Hangovers are caused by drinking too much alcohol. A single alcoholic drink is enough to trigger a hangover for some people, while others may drink heavily and escape a hangover entirely.

Various factors may contribute to a hangover. For example:

• Alcohol causes your body to produce more urine. In turn, urinating more than usual can lead to dehydration — often indicated by thirst, dizziness and lightheadedness.
• Alcohol triggers an inflammatory response from your immune system. Your immune system may trigger certain agents that commonly produce physical symptoms, such as an inability to concentrate, memory problems, decreased appetite and loss of interest in usual activities.
• Alcohol irritates the lining of your stomach. Alcohol increases the production of stomach acid and delays stomach emptying. Any of these factors can cause abdominal pain, nausea or vomiting.
• Alcohol can cause your blood sugar to fall. If your blood sugar dips too low, you may experience fatigue, weakness, shakiness, mood disturbances and even seizures.
• Alcohol causes your blood vessels to expand, which can lead to headaches.
• Alcohol can make you sleepy, but it prevents deeper stages of sleep and often causes awakening in the middle of the night. This may leave you groggy and tired.

Congeners

Alcoholic beverages contain ingredients called congeners, which give many types of alcoholic beverages their flavor and can contribute to hangovers. Congeners are found in larger amounts in dark liquors, such as brandy and bourbon, than in clear liquors, such as vodka and gin.

Congeners are more likely to produce a hangover or increase the severity of a hangover. But drinking too much alcohol of any color can still make you feel bad the next morning.