It’s a popular yet unproven belief that dehydration is one of the main causes of hangover. While dehydration can contribute to symptoms of a hangover, it may not be the cause of hangovers per se — see below.
Rather, hangover seems to be the result of the direct effects of alcohol (ethanol) on the production of reactive oxygen species (ROS) — which are generated during the breakdown of alcohol. ROS elicit mitochondria damage and oxidative stress throughout the body and central nervous system (CNS). ROS also activate the immune system, initiating both pro- and anti-inflammatory cytokine responses in the body and CNS. Symptoms such as nausea, vomiting, headache, confusion, tremor, and learning and memory deficits occur in the presence of elevated levels of inflammatory cytokines, and higher levels of inflammation and ROS are associated with a greater hangover severity, directly implicating them in the pathology of hangover.[1]
Other factors contributing to hangover include: an alteration in neurotransmitters such as GABA, glutamate, dopamine, serotonin, and the endocannabinoids; disturbances in sleep quality or quantity due to alcohol consumption; and alterations in hormones and electrolytes.[2]