Among the most common provocatives of dreams are sensations connected with a difficulty in breathing, due to the closeness of the air or to the pressure of the bed-clothes on the mouth.

J.Boerner investigated the influence of these circumstances by covering with the bed-clothes the mouth and a part of the nostrils of persons who were sound asleep.

This was followed by a protraction of the act of breathing, a reddening of the face, efforts to throw off the clothes, etc.

On being roused, the sleeper testified that he had experienced a nightmare, in which a horrid animal seemed to be weighing him down.[87] Irregularity of the heart's action is also a frequent cause of dreams.

It is not improbable that the familiar dream-experience of flying arises from disturbances of the respiratory and circulatory movements.
Again, the effects of indigestion, and more particularly stomachic derangement, on dreams are too well known to require illustration.