Mary Shelley acknowledged in her Introduction to the revised edition of Frankenstein of 1831, that the question was "so very frequently asked me--'How I, then a young girl, came to think of, and to dilate upon, so very hideous an idea?'"

Mary Shelley then points to an event of a specific evening—June 21, 1816—as the moment that the core of her novel emblazoned itself in her mind. In that introduction written over 15 years later, she tells a story almost as well-known as the novel itself, of how she and Byron and Percy Shelley and Dr. Polidori, after reading ghost stories together one rainy evening near Geneva in June, 1816, agreed each to write an equally thrilling horror story; how she tried for days to think of a story, but failed; and finally, how one night (June 21) after a discussion among Byron, Polidori, and Percy Shelley concerning galvanism and Erasmus Darwin's success in causing a piece of vermicelli to move voluntarily, she fell into a reverie or waking dream in which she saw "the pale student of unhallowed arts kneeling beside the thing he had put together" and felt the terror he felt as the hideous corpse he had reanimated with a "spark of life" stood beside his bed, "looking on him with yellow, watery, but speculative eyes."

Why did Mary Shelley have such a dream at this point in her life? Affectively, the dream evoked a powerful anxiety in her. Over fifteen years later, she claimed she could still see vividly the room to which she woke and feel "the thrill of fear" that ran through her. Why was she so frightened? Remember that Mary Shelley had given birth to a baby girl eighteen months earlier, a baby whose death two weeks later produced a recurrent dream: "Dream that my little baby came to life again; that it had only been cold, and that we rubbed it before the fire' and it lived. Awake and find no baby." Once again she was dreaming of reanimating a corpse by warming it with a "spark of life." And only six months before, Mary Shelley had given birth a second time, to William. She doubtless expected to be pregnant again in the near future; and indeed, she conceived her third child, Clara Everina, only six months later in December. Mary Shelley's reverie unleashed her deepest subconscious anxieties, the natural but no less powerful anxieties of a very young, frequently pregnant woman. (Mellor 40)

Of course, many other factors and circumstances influenced her state of mind to produce the nightmare. We might also note that the nightmare motif made a significant impact on her. In 1792 her mother Mary Wollstonecraft offered to enter a ménage a trois with the Romantic painter and intellectual Henri Fuseli and his wife, but was refused. One of Fuseli’s most memorable and striking paintings is entitled "The Nighmare." Anne Mellor observes that Mary knew the painting very well, and that several critics have suggested that the description in Frankenstein of the death of Elizabeth Lavenza on her wedding night is based on it (121).