As someone who experiences them, Sally Adee explores their causes:
It goes by many names, known variously as night terrors, the incubus, witches’ pressure and Old Hag syndrome. It happens when a few important wires get crossed in your brain and you accidentally wake up in the middle of dreaming. Old Hag syndrome is usually explained away as an evolutionary hiccup, a terrifying but harmless side effect of a mechanism that evolved to protect you. …
Waking sleep paralysis is a glitch where you wake up in the middle of REM sleep. During the REM phase — the part of the sleep cycle when you do your dreaming — your body and evolution have wisely conspired to paralyse you. Well played, evolution: Considering the crazy shit most of us get up to in our dreams, it was a good idea to equip our motor cortex with an off switch. Before this feature came standard-issue, several of our ancestors undoubtedly went off on lovely dream-fuelled midnight excursions that terminated in the stomach of a bear.
Last September, Alexis Madrigal reported on the work of Shelley Adler, who found that in the 1980s Hmong men of a certain age "were in some sense killed by their powerful cultural belief in night spirits":
The truth is that we don't understand the relationship between belief and biology quite as well as we'd like to think. That's one reason sleep paralysis is so useful as a probe for the boundary of mind and body. The night-mare is "a link between our biological and cultural selves." While people of all cultures experience sleep paralysis in similar ways, the specific form and intensity it takes varies by culture, soaking up whatever local spirits or monsters happen to be lurking nearby.
The Dish 