The alcohol hangover develops when blood alcohol concentration (BAC) returns to zero and is characterized by a feeling of general misery that may last more than 24 h. It comprises a variety of symptoms including drowsiness, concentration problems, dry mouth, dizziness, gastro-intestinal complaints, sweating, nausea, hyper-excitability, and anxiety. The alcohol hangover is an intriguing issue since it is unknown why these symptoms are present after alcohol and its metabolites are eliminated from the body.

Although numerous scientific papers cover the acute effects of alcohol consumption, researchers largely neglected the issue of alcohol hangover. This lack of scientific interest is remarkable, since almost everybody is familiar with the unpleasant hangover effects that may arise the day after an evening of excessive drinking, and with the ways these symptoms may affect performance of planned activities.

Many people favour the (unproven) popular belief that dehydration is the main cause of alcohol hangover symptoms. However, taking a closer look at the present research on biological changes during alcohol hangovers suggests otherwise.
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nterestingly, no significant differences were found in absenteeism between workers reporting hangovers and those who did not. A possible explanation may be that workers with a hangover feel that having a hangover is ‘their own fault’, and the obligation they have to go to work may prevent calling sick. The fact that workers do go to work when having a hangover is of concern, especially since some in jobs making the wrong decisions may have serious consequences.

The article by Stephens and colleagues calls for additional hangover research, using more sophisticated research methods. In this context, researchers should ask themselves the question ‘ what is the alcohol hangover?’. It is evident that besides the alcohol amount many other factors play a role in determining the presence and severity of hangovers. To complicate matters, co-occurring dehydration and sleep deprivation have an impact on the next-day effect of excessive alcohol consumption as well. Until future research elucidates its pathology, the alcohol hangover remains a puzzling phenomenon.

Source: alcohol hangover–a puzzling phenomenon | Alcohol and Alcoholism | Oxford Academic

It turns out we don’t really know much about hangovers and it’s quite difficult to actually study them.