Light and Human Health #4 – When We Lost the Light: The Industrial Revolution and Health

The Birth of the Diseases of Darkness

The consequences of this lifestyle shift appeared quickly and on a massive scale.

Rickets

For the first time in European history, entire populations of children with bone deformities were observed. Rickets became a distinctly urban disease, linked to the lack of sunlight, fresh air, and adequate living conditions. In some working-class districts, symptoms of rickets were present in the majority of children.

Tuberculosis

Damp, dark housing conditions promoted the spread of tuberculosis. Physicians observed that patients often improved after leaving the city, travelling to the mountains or the seaside, places with more light and fresh air.

Exhaustion and Mood Disorders

Historical sources from the period describe apathy, chronic fatigue, insomnia, and weakened immunity. Today, we would recognise these symptoms as circadian rhythm disruption and light deficiency. At the time, they were poorly understood and difficult to treat.

Early Attempts to Restore Health Through Light

Although the biological mechanisms of light were not yet known, physicians intuitively returned to long-standing recommendations: walking in daylight, travelling “for health” outside the city, opening windows, and improving ventilation.

During this period, European health resorts began to develop, along with the first principles of health-oriented architecture. These efforts, however, were local and fragmented rather than systemic.

Industrial-Era Hospitals: Places Without Light

The conditions of industrial cities were reflected in hospitals. Before the mid-19th century, hospitals were overcrowded, poorly ventilated, dark, and conducive to the spread of infections.

Light was not considered a therapeutic element. Like fresh air and space, it remained largely outside the focus of medical practice.

And it was precisely in this world that a figure emerged who would change the course of history.

Light That Had Not Yet Been Named

Nineteenth-century physicians observed the effects of sunlight deprivation, but they did not yet understand its biological mechanisms. They did not know what vitamin D was, nor did they understand UVB radiation or cutaneous synthesis processes. And yet, their observations were remarkably accurate.

Rickets, bone deformities, and increased susceptibility to infections appeared on a mass scale only in the industrial cities. It was not until 1922 that science gave this deficiency a name: vitamin D. As Professor Carsten Carlberg emphasises, this marked the first time in history that vitamin D deficiency became a population-wide problem.

For hundreds of thousands of years of evolution, humans lived in the light. Modernity was the first era to strip them of this biological foundation.

Florence Nightingale and the Return of Light to Medicine

It was within the dark, overcrowded hospitals of the 19th century that Florence Nightingale emerged.

She did not yet know the concept of vitamin D, but she saw what ancient physicians had understood: the absence of light worsened patients’ conditions, while its presence supported recovery. In Notes on Nursing (1860), she wrote:

“Second only to their need of fresh air is their need of light… and it is not only light but direct sunlight they want.”

Nightingale was the first to reintroduce light into modern medicine, not as intuition or tradition, but as a foundational element of systematic healthcare. Opening windows, orienting buildings toward daylight, providing space, fresh air, and access to sunlight became the cornerstones of her reforms.

From that moment on, the history of light in medicine entered a new chapter.

What’s Next in the “Light and Human Health” Series?

In the next part, we will take a closer look at the work of Florence Nightingale: her observations, hospital reforms, and how her approach anticipated modern chronobiology by more than a century. The story of sunlight in medicine continues.