The Nightmare shows a woman in deep sleep with her arms thrown below her, and with a demonic and apelike incubus crouched on her chest.
The painting's dreamlike and haunting erotic evocation of infatuation and obsession was a huge popular success. After its first exhibition, at the 1782 Royal Academy of London, critics and patrons reacted with horrified fascination and the work became widely popular, to the extent that it was parodied in political satire and an engraved version was widely distributed. In response, Fuseli produced at least three other versions.
Interpretations vary. The canvas seems to portray simultaneously a dreaming woman and the content of her nightmare. The incubus and horse's head refer to contemporary belief and folklore about nightmares, but have been ascribed more specific meanings by some theorists. Contemporary critics were taken aback by the overt sexuality of the painting, since interpreted by some scholars as anticipating Jungian ideas about the unconscious.
The Nightmare simultaneously offers both the image of a dream—by indicating the effect of the nightmare on the woman—and a dream image—in symbolically portraying the sleeping vision. It depicts a sleeping woman draped over the end of a bed with her head hanging down, exposing her long neck. She is surmounted by an incubus that peers out at the viewer. The sleeper seems lifeless and, lying on her back, takes a position then believed to encourage nightmares. Her brilliant coloration is set against the darker reds, yellows, and ochres of the background; Fuseli used a chiaroscuro effect to create strong contrasts between light and shade. The interior is contemporary and fashionable and contains a small table on which rests a mirror, phial, and book. The room is hung with red velvet curtains which drape behind the bed. Emerging from a parting in the curtain is the head of a horse with bold, featureless eyes.
For contemporary viewers, The Nightmare invoked the relationship of the incubus and the horse (mare) to nightmares. The work was likely inspired by the waking dreams experienced by Fuseli and his contemporaries, who found that these experiences related to folkloric beliefs like the Germanic tales about demons and witches that possessed people who slept alone. In these stories, men were visited by horses or hags, giving rise to the terms "hag-riding" and "mare-riding," and women were believed to engage in sex with the devil. The etymology of the word "nightmare", however, does not relate to horses. Rather, the word is derived from mara, a Scandinavian mythological term referring to a spirit sent to torment or suffocate sleepers. The early meaning of "nightmare" included the sleeper's experience of weight on the chest combined with sleep paralysis, dyspnea, or a feeling of dread. The painting incorporates a variety of imagery associated with these ideas, depicting a mare's head and a demon crouched atop the woman.
Sleep and dreams were common subjects for the Zürich-born Henry Fuseli, though The Nightmare is unique among his paintings for its lack of reference to literary or religious themes (Fuseli was an ordained minister).
...Contemporary critics often found the work scandalous due to its sexual themes. A few years earlier Fuseli had fallen for a woman named Anna Landholdt in Zürich, while travelling from Rome to London. Landholdt was the niece of his friend, the Swiss physiognomist Johann Kaspar Lavater. Fuseli wrote of his fantasies to Lavater in 1779; "Last night I had her in bed with me—tossed my bedclothes hugger-mugger—wound my hot and tight-clasped hands about her—fused her body and soul together with my own—poured into her my spirit, breath and strength. Anyone who touches her now commits adultery and incest! She is mine, and I am hers. And have her I will.…"
Fuseli's marriage proposal met with disapproval from Landholdt's father, and in any case seems to have been unrequited—she married a family friend soon after. The Nightmare, then, can be seen as a personal portrayal of the erotic aspects of love lost. Art historian H. W. Janson suggests that the sleeping woman represents Landholdt and that the demon is Fuseli himself. Bolstering this claim is an unfinished portrait of a girl on the back of the painting's canvas, which may portray Landholdt. Anthropologist Charles Stewart characterizes the sleeping woman as "voluptuous," and one scholar of the Gothic describes her as lying in a "sexually receptive position." In Woman as Sex Object (1972), Marcia Allentuck similarly argues that the painting's intent is to show female orgasm. This is supported by Fuseli's sexually overt and even pornographic private drawings (e.g., Symplegma of Man with Two Women, 1770–78). Fuseli's painting has been considered representative of sublimated sexual instincts. Related interpretations of the painting view the incubus as a dream symbol of male libido, with the sexual act represented by the horse's intrusion through the curtain. Fuseli himself provided no commentary on his painting.