"Unity Kinkaid finds it harder and harder to stay awake. She now sleeps for almost 20 hours a day. She used to dream; to shift in her sleep, muttering and sighing, locked in half-remembered fantasies. Now she lies unmoving, breath shallow and silent, lost to the world. Unity sleeps." – The Sandman.
At the beginning of Neil Gaiman's The Sandman comics, and the wildly popular Netflix series based on it, the narrator describes a "sleeping sickness" that spreads around the world after the Lord of Dreams, aka Morpheus, becomes trapped in the waking world by a wizard's spell.
The people affected by Dream's capture sleep for years, or else wander around in half-sleep, often dying before they are able to wake. The disease named in The Sandman is Encephalitis lethargica. You'll be shocked to find out a science website is not about to explain that the anthropomorphic entity of sleep being captured by a wizard was the cause of it, but the disease was real, affected millions in the 1910s and '20s, and to this day has not been satisfactorily explained.
"They would be conscious and aware – yet not fully awake; they would sit motionless and speechless all day in their chairs, totally lacking energy, impetus, initiative, motive, appetite, affect or desire; they registered what went on about them without active attention, and with profound indifference," Dr Oliver Sacks, who treated patients for the disease, wrote of his time spent at the Beth Abraham Hospital in New York in the 1960s.
"They neither conveyed nor felt the feeling of life; they were as insubstantial as ghosts and as passive as zombies."
The first cases of the disease were reported in 1916–17 Europe, coinciding with the Spanish Flu in 1918, and continuing into the 1930s. More than one million people worldwide are thought to have caught the disease, which thankfully now only occurs rarely.
Patients would initially present with flu-like symptoms, as well as fatigue, fever and vomiting.
"Neurological symptoms followed and could present very quickly, as in the case of a girl who experienced a sudden hemiplegia while walking home from a concert. Within half an hour she was asleep, and died 12 days later," Leslie Hoffman and Joel Vilensky wrote in a 2017 paper on the century after the epidemic.
Shortly after this, patients would become dazed and delirious. They would sleep for abnormally long periods, but would be aware of events that had happened around them while they were in this state of semi-sleep. Patients with these symptoms had a less than 50/50 chance of survival. Patients with a different form of the disease could expect rigidity in their limbs, and to be immobile for long periods of time.
Those that did survive were not out of the woods, as complications often set in years after the initial infection.
"The chronic phase was characterized by parkinsonism, but sleep disturbances, oculomotor abnormalities, involuntary movements, speech and respiratory abnormalities, and psychiatric disorders were also common features," Hoffmann and Vilensky wrote. "In the decades following the epidemic, it was estimated that as many as 50% of parkinsonism cases were postencephalitic."
The cause of the disease remains, largely, unknown. One theory is that the disease was caused by the virus responsible for the 1918 flu pandemic, which arrived and ended with similar timings. More recently, a study looking at the few available brain tissue samples from patients concluded that an enterovirus – a group of viruses that includes polio, spread through coming into contact with secretions from an infected patient – was the likely cause of the disease.