An epidemic caused people to fall asleep for months – we still don’t know why

It was also deadly.

One hundred years ago, across the world, people were falling asleep uncontrollably.

Not from a hard day’s work or a late night, but a disease known as ‘sleepy sickness’.

Victims fell into a slumber so deep that those who caught it often didn’t wake for weeks, or even months, at a time. It was also deadly, killing 30 to 40% of those affected, usually from respiratory failure.

An epidemic, it emerged from northern France in 1916, spreading first across Europe, and then to North America, Central America and India, before disappearing almost entirely by 1930.

To this day, no one knows exactly how it spread, what caused it – or if the disease could return.

Called encephalitis lethargica (EL), it first caused flu-like symptoms, including a headache, nausea, joint pain and a fever. From there, it spread to the eyes, which became uncoordinated, leading to double vision. The eyelids then began to droop, and patients were overwhelmed with the need to sleep, day or night.

Although they could often be woken, patients would quickly fall back asleep, many experiencing vivid dreams or nightmares as they did. Some, however, were comatose.

In a magazine article uncovered by Scientific American, Eleanore Carey, a young woman living in New York, described the disease as like ‘being buried in a pit as deep as the centre of the Earth’.

‘After two months of illness I was in little pain, in fact, I was very comfortable, provided they did not prod me nor stand me on my head, turn me over in bed nor dash cold water on my face to waken me,’ she said. 

‘It was so heavenly just to be allowed to sleep, but these people around me seemed determined to prevent my being comfortable! When the idea finally crept through my sleeping brain that I must waken, it seemed to be a physical impossibility. I wanted to be obliging, but I just could not.

‘It seemed to me to be just as difficult to come to consciousness as it would have been had I been buried in a pit as deep as the center [sic] of the earth, where the circular walls about me were of shiny, polished marble.’

Another girl was not so lucky. While walking home from a concert one side of her body suddenly became paralysed, and she soon fell into a deep sleep. She died 12 days later.

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At the Derby and Derbyshire Rescue and Training Home (for girls), 12 of 21 girls and women in the house fell victim to EL in a fortnight. Half died within 10 days of symptoms appearing.

The case is one in support of the disease being contagious, but many more had no such link.

Of 1,156 cases in Vienna, 520 in Germany and 464 in France, no evidence of direct transmission was found – but natural immunity to the disease could not be ruled out, allowing ‘healthy’ people to spread the disease without realising.

The number of patients thought to have been affected by EL varies from 52,000 to one million, but despite the high number, doctors were at a loss as to what was causing their endless sleep.

In the 1920s, more than 2,000 scientific articles were written about the disease, but no one found a definitive answer.

Its appearance around the same time as the Spanish flu pandemic led some to suggest it may be linked to the H1N1 virus that killed up to 50 million people across the globe.

But analysis then and more recently, thanks to well-preserved brain samples, has shown no evidence of a bacteria or virus that might trigger such a reaction.

Others argue it could be an autoimmune response, an overreaction by the body triggered by a cold or flu virus, rather than a result of the infection itself.

A reasonable explanation, but not one that answers how it could appear and disappear so quickly, sweeping entire countries before evaporating.

Sadly however, while the disease itself stopped spreading, for those infected life did not return to normal. 

Many developed Parkinson’s disease, with symptoms ranging from tremors to losing the ability to move their muscles at all, becoming ‘frozen’.

Known as post-encephalitic parkinsonism (PEP), in younger patients it could also cause changes in character and behaviour, from clinginess and impaired concentration while younger, to violence in adolescence.

As adults, those whose PEP did not leave them hospital-bound often found themselves living a life of crime, including committing violent acts such as rape and murder.

Many referred to it as ‘a disease that makes criminals’.

Some scientists believe Hitler may have had PEP, suspecting he suffered from EL while in hospital recovering from poison gas aged 29.

Autopsies and analysis of those with EL and PEP have revealed damage in a particular region of the brain, the basal ganglia, which – among other functions – controls movement and the processing of emotions.

With few cases of EL since, there has been little opportunity to study the disease, or figure out if it could ever make a return. 

In 2001, the case of a woman who died following 12 months of EL was published, and again revealed swelling of the brain, but no virus or other obvious cause.

Now, a century after the epidemic, scientists are no closer to solving the mystery of sleepy sickness – a disease much more dangerous than it sounds.