Depends on who you ask. While there could potentially be malevolent spirits that attack us while we sleep, there are more mundane explinations for the so called "incubus phenomenon". (Oh, and for the record, the plural of incubus is incubi. Its Latin).
Couple real quick definitions:
- Incubus: a male demon that attacks women in their sleep
- Succubus: a female demon that attacks men in their sleep
It is believed that the Incubus (or Succubus) myth comes from a strange biological event that occurs while we are sleeping. As the body drifts into deeper stages of sleep, a chemical is released which is supposed to paralyse the body (this prevents us from kicking, screaming, and otherwise injuring ourselves during dreams). In some people, their body produces too much of this chemical and so it does not burn off by the time the person awakes, or, alternately, someone may wake suddenly (from a bad dream, for instance) with the chemical still in their system and be unable to move.
This, coupled with superstition, explains the most terrifying part of the encounter with the Incubus, but there are other factors to consider as well.
- People's depth of sleep regularly cycles. It is entirely possible that when you "woke up" your brain was still partially in its sleep cycle. This can cause hallucinations (both visual and auditory).
- It is possible that you did just dream the whole thing
- If you wake up briefly in the middle of a sleep cycle but fall asleep again fast enough, upon waking the next morning (if you remember anything at all), its possible you "remembered" two events as having happened simultaneously that didn't.