A fiend or incubus formerly supposed to cause trouble in sleep.
A condition in sleep usually caused by improper eating or by digestive or nervous troubles, and characterized by a sense of extreme uneasiness or discomfort (as of weight on the chest or stomach, impossibility of motion or speech, etc.), or by frightful or oppressive dreams, from which one wakes after extreme anxiety, in a troubled state of mind; incubus.
Hence, any overwhelming, oppressive, or stupefying influence.
Teamwork makes the dream work, but a vision becomes a nightmare when the leader has a big dream and a bad team. John C. Maxwell
I always wished for this, but it's almost turning into more of a nightmare than a dream. Eminem
We are weighed down, every moment, by the conception and the sensation of Time. And there are but two means of escaping and forgetting this nightmare: pleasure and work. Pleasure consumes us. Work strengthens us. Let us choose. Charles Baudelaire