Researchers at Brown University liquored up nearly a hundred study participants with various alcoholic drinks—all the different shades of alcohol were mixed into caffeine-free cola to mask the color and flavor of the particular alcohol—and observed them. All the participants that consumed alcohol slept worse and performed worse on cognitive tests than the participants that didn’t, but the participants that drank lighter-colored liquor had less hangover related symptoms the next day.

Overall, bourbon drinkers reported feeling worse than vodka drinkers, rating higher on scales that measure the severity of hangover malaise, including headache, nausea, loss of appetite and thirst. It should come as no surprise that alcohol drinkers said they felt much worse than those who had drunk only tonic water.

One reason for the different effects of vodka and bourbon, Rohsenow says, could be that bourbon contains 37 times more toxic compounds than vodka does, including nasty organic molecules such as acetone, acetaldehyde, tannins and furfural. A good rule of thumb for liquors, she says, is that the clearer they are, the less of these substances they contain.