According to medieval legend, a succubus was an evil, demonic woman who would attack and seduce men while they slept; an incubus was a demonic man who would attack and seduce women. These legends were fueled by actual experiences that men and women frequently had; they would wake up in the middle of the night, but be unable to move. They would feel pressure of their chest, as if something was on top of them, and they would experience a feeling of extreme panic. It seemed obvious that they were being attacked by demons.
However, a common sleep disorder called night terrors or sleep terrors produce the same exact experience that was described above. When people experience night terrors, they are actually in a deep sleep, even though they may think they are awake; this is why they cannot move. Also, because they are asleep, they can produce dreams to explain the sensations they are experiencing. The subject of the dream depends on the beliefs of the person. People who believe in demons will imagine demons. People who believe in aliens will imagine aliens; night terrors are likely involved in many alien abduction stories.
The legends of succubi and incubi might also have been used to explain more common, albeit unseemly, matters. According to legend, succubi were especially prone to attacking religious men in their sleep, and incubi could even impregnate women. The myths could have been used simply to account for inappropriate, sexual dreams that priests and monks had, as well as pregnancies that occurred outside of marriage.
Creatures of the Night
Some legends describe humans who were turned into monsters. Many of these myths might actually be based on diseases that cause humans to display odd behaviors and physically characteristics.
Porphyria is a group of several genetic disorders involving enzyme deficiencies. Porphyria causes a range or bizarre symptoms, including paranoia, hallucinations, photosensitivity, and ulcers that can cause deformities. Occasionally, victims of porphyria may also have red or purple urine. Some of these symptoms match the descriptions of vampires: the mental disturbances and physical deformities could make porphyria sufferers seem like insane monsters; the photosensitivity would make porphyria sufferers avoid the sun and even blister severely when they are exposed to too much sunlight; red urine could lead people to think that porphyria sufferers drank blood. Although this biological explanation is possibly at least partially responsible for vampire legends, humanity’s fear of death and the unknown probably contributed significantly to the shaping of these myths. Porphyria has also been connected to legends of werewolves.