Despite working during the height of the Enlightenment and the so-called "Age of Reason," Swiss-English painter, Henry Fuseli, instead chose to depict darker, irrational forces in this masterpiece. The Nightmare became an icon of Romanticism and a defining image of Gothic horror.  The painting has yielded many interpretations and is seen as a precursor to late nineteenth-century psychoanalytic theories regarding dreams and the unconscious. The father of psychoanalysis himself, Sigmund Freud, studied this painting and kept a print in his office.

The figure that sits atop the woman's chest is often described as an "imp" or an "incubus." This type of malevolent spirit from germanic legend was believes to lie upon people in their sleep or even have sexual intercourse with sleeping women. The presence of the imp may be understood as a mythic representation of the physical experience of chest pressure during sleep paralysis. Although it is tempting to understand the painting's title as a reference to the horse in the background, the term "nightmare" does not refer to the equine figure. Instead, the name refers to the antiquated term "mara," which refers to an evil spirit that tortured humans by suffocating them in their sleep. It is possible that the milky eyed horse is the evil steed of the mischievous incubus.

Fuseli's painting is suggestive, but not explicit, leaving open the possibility that the woman is simply dreaming. This dream has taken form in a frightening and physical manifestation of the demonic figures. However, the violent slash marks on the drapery suggests that she may have succumbed to the terrors and died. The table top on the left side of the painting holds some mysterious bottles. Perhaps they are perfumes or medicinal concoctions. More than likely, the glasses contain laudanum, a narcotic mixture of alcohol and opium that was popular during Fuseli's time.

Through his use of composition and chiaroscuro –the strategic contrasting of light and shadow– Fuseli heightens the drama and uncertainty of his scene. Traditionally, chiaroscuro was used to symbolize the literal enlightening power of rational observation, However, Fuseli's work instead reveals the futility of such light to penetrate and explain the darker realms of the unconscious. The single light source coming from the right, the curtains and tassels in the background, and the shorted foreground all contribute to the theatricality of the work. The red drapery falling off the edge of the bed suggests a river of blood as it might be enacted on stage in the operatic performances of the time.