People with central hypoventilation syndrome, also known as Ondine’s curse, can forget to breathe.

The autonomic nervous system is the autopilot brain. It keeps our essential systems running: heart rate, drooling, breathing, pretty much all of the automatic actions that our bodies need to survive. It’s the reason that people without higher brain function, like those in comas, keep on digesting their food and pooping it out. But, for some people, the autonomic nervous system malfunctions. This is central hypoventilation syndrome.
So, remember that post a couple of years ago about the elementals of the 16th century alchemist Paracelsus? Paracelsus created the undine, fictional water nymphs. In 1811 a German author named Friedrich de la Motte Fouqué wrote a novella about a romantic entanglement between a knight and a nymph, Ondine. This went on to inspire numerous paintings, sculptures, songs, ballets, operas, films, books, and stories (including, by the way, Hans Christian Andersen’s The Little Mermaid).
In 1938, a French dramatist wrote a play based on Fouqué’s story, Ondine. In it, a knight marries the water spirit, and she pledges to “be the breath of your lungs.” He, of course, cheats on her, and a strange curse falls upon him:
Since you left me, Ondine, all the things my body once did by itself, it does now only by special order. […] I have to supervise five senses, two hundred bones, a thousand muscles. A single moment of inattention, and I forget to breathe. He died, they will say, because it was a nuisance to breathe.
Ondine
This, then, is Ondine’s curse. You must breathe, not automatically, but by conscious choice. Central hypoventilation syndrome afflicts some people whose autonomic nervous system has been damaged, either by injury or congenitally. They must remember to breathe. When they sleep, they risk damage or death from sleep apnea. Fortunately, once diagnosed people can wear special equipment to keep them breathing at night.