Nightmare, Or Incubus, an affection coming on during sleep, in which there is a sense of great pressure upon the chest accompanied by inability to move. It is well known that uneasy or painful sensations occurring during sleep, when insufficient to wake the patient directly, are apt to produce disagreeable or frightful dreams. Thus persons laboring under a variety of chronic affections, particularly heart disease, upon falling asleep suffer from dreadful illusions; they are attacked by wild beasts or armed men, they are falling down precipices, etc. The same phenomenon often occurs during fever or after mental excitement attended with anxiety. The more common form of nightmare usually comes on during the first sleep, and is caused by a constrained position or by a distended stomach, which act by somewhat impeding respiration. The feeling of oppression thus caused gives rise to the nightmare, and is dispelled at once when the patient awakes, and, bringing the voluntary muscles into play, draws a full breath.