From The New Yorker:
Hangovers also have an emotional component. Kingsley Amis, who was, in his own words, one of the foremost drunks of his time, and who wrote three books on drinking, described this phenomenon as “the metaphysical hangover”: “When that ineffable compound of depression, sadness (these two are not the same), anxiety, self-hatred, sense of failure and fear for the future begins to steal over you, start telling yourself that what you have is a hangover. . . . You have not suffered a minor brain lesion, you are not all that bad at your job, your family and friends are not leagued in a conspiracy of barely maintained silence about what a shit you are, you have not come at last to see life as it really is.” Some people are unable to convince themselves of this. Amis described the opening of Kafka’s “Metamorphosis,” with the hero discovering that he has been changed into a bug, as the best literary representation of a hangover.