Why do we get hangovers?

Alcohol suppresses vasopressin, the hormone that tells the kidneys to hold on to water, so you urinate more and wake up dehydrated with a headache. The liver converts alcohol into acetaldehyde, a toxic compound that contributes to nausea and a racing pulse before it is broken down further. Alcohol also fragments sleep and cuts REM sleep, which is why you feel exhausted even after a long night in bed. Darker drinks contain more congeners, by-products of fermentation, and a 2010 Brown University study found bourbon caused worse hangovers than vodka at the same alcohol dose. Symptoms usually peak as blood alcohol returns to zero, and despite countless remedies, only time truly cures a hangover.