An oneirogen, from the Greek ὄνειρος óneiros meaning "dream" and gen "to create", is a substance or other stimulus which produces or enhances dreamlike states of consciousness. This is characterized by an immersive dream state similar to REM sleep, which can range from realistic to alien or abstract.
Many dream-enhancing plants such as dream herb (Calea zacatechichi) and African dream herb (Entada rheedii), as well as the hallucinogenic diviner's sage (Salvia divinorum), have been used for thousands of years in a form of divination through dreams, called oneiromancy, in which practitioners seek to receive psychic or prophetic information during dream states. The term oneirogen commonly describes a wide array of psychoactive plants and chemicals ranging from normal dream enhancers to intense dissociative or deliriant drugs.
Effects experienced with the use of oneirogens may include microsleep, hypnagogia, fugue states, rapid eye movement sleep (REM), hypnic jerks, lucid dreams, and out-of-body experiences. Some oneirogenic substances are said to have little to no effect on waking consciousness, and will not exhibit their effects until the user falls into a natural sleep state.