The Nightmare (1781) by Henry Fuseli

A 1781 oil painting by Henry Fuseli (1741-1825) influenced many writers like William Blake, Mary Shelley, and Edgar Allan Poe whose narrator in "The Fall of the House of Usher" (1839) says, 

An irrepressible tremour gradually pervaded my frame; and, at length, there sat upon my very heart an incubus of utterly causeless alarm. Shaking this off with a gasp and a struggle, I uplifted myself upon the pillows, and, peering earnestly within the intense darkness of the chamber, hearkened --I know not why, except that an instinctive spirit prompted me --to certain low and indefinite sounds which came, through the pauses of the storm, at long intervals, I knew not whence. Overpowered by an intense sentiment of horror, unaccountable yet unendurable, I threw on my clothes with haste (for I felt that I should sleep no more during the night), and endeavoured to arouse myself from the pitiable condition into which I had fallen, by pacing rapidly to and fro through the apartment.

According to Wikipedia, "Poe and Fuseli shared an interest in the subconscious; Fuseli is often quoted as saying, 'One of the most unexplored regions of art are dreams.'"