“Alcohol hangover is characterized by adverse physical and mental effects that occur the next morning after the intake of toxic doses of alcohol. This phenomenon is a very frequent experience among alcohol-consuming people and it has been associated to high socio-economic costs, mainly due to absenteeism and poor performance in academic and work settings. Multiple theories exist to explain the appearance and severity of hangover, such as direct or indirect effects of alcohol, alcohol withdrawal, and toxic effects of alcoholic beverage congeners or of alcohol metabolites. In addition, a number of intervening factors have been identified, such as vulnerability to alcohol dependence or psychological factors.
Although several studies analyze and describe hangover, it is still poorly understood. Further well-designed studies with a unitary methodology and clear operational criteria to define hangover are necessary in order to clarify such a phenomenon
source: Alcohol hangover: a critical review of explanatory factors in: Human Psychopharmacology: Clinical and Experimental, Volume 24, Issue 4