To dream that you have a hangover suggests that you are heading in the wrong direction. You need to utilize your mind more, and not let your emotions get out of control.
Dreams of a hangover represent exhaustion as you burn the candle at both ends; you are dealing with the consequences for ingesting information or the overindulgence of non-supportive people or habits in your life. It is time for some changes in your lifestyle. See Drunk and Alcohol.