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For millennia, humans had one obvious and reliable source of light – the Sun – and we knew the Sun was essential for our survival.

This might be why ancient religions – such as those in , , , , , and and – involved Sun worship.

Early religions were also often . Sick people would turn to the shaman, priest or priestess for help.

While ancient peoples used the Sun to heal, this might not be how you think.

Since then, we’ve used light to heal in a number of ways. Some you might recognise today, others sound more like magic.

From warming ointments to sunbaking

There’s not much evidence around today that ancient peoples believed sunlight itself could cure illness. Instead, there’s more evidence they used the warmth of the Sun to heal.

The Ebers Papyrus is an ancient Egyptian medical scroll from around 1500 BCE. It contains a recipe for an ointment to ” “. The ointment was made of wine, onion, soot, fruit and the tree extracts frankincense and myrrh. Once it was applied, the person was “put in sunlight”.

Other recipes, to treat coughs for example, putting ingredients in a vessel and letting it stand in sunlight. This is presumably to warm it up and help it infuse more strongly. is in the medical writings attributed to who lived around 450-380 BCE.

The physician Aretaeus, who was active around 150 CE in what is now modern Turkey, wrote that chronic cases of what he called “lethargy” but we’d recognise today as depression:

Lethargics are to be laid in the light, and exposed to the rays of the Sun (for the disease is gloom); and in a rather warm place, for the cause is a congelation of the innate heat.

Classical Islamic scholar Ibn Sina (980-1037 CE) described the health effects of sunbathing (at a time where we didn’t know about the link to skin cancer). In Book I of he said the hot Sun helped everything from flatulence and asthma to hysteria. He also said the Sun “invigorates the brain” and is beneficial for “clearing the uterus”.

It was sometimes hard to tell science from magic

All the ways of curing described so far depend more on the Sun’s heat rather than its light. But what about curing with light itself?

English scientist Sir Isaac Newton (1642-1727) knew you could “split” sunlight into a .

This and many other discoveries radically changed ideas about healing in the next 200 years.

But as new ideas flourished, it was sometimes hard to tell .

For example, German mystic and visionary Jakob Lorber (1800-1864) believed sunlight was the best cure for pretty much anything. His 1851 book The Healing Power of Sunlight was .

Public health reformer Florence Nightingale (1820-1910) in the power of sunlight. In her famous book , she said of her patients:

second only to their need of fresh air is their need for light […] not only light but direct sunlight.

Nightingale also believed sunlight was the natural enemy of bacteria and viruses. She seems . Sunlight can kill some, but not all, bacteria and viruses.

Chromotherapy – a way of healing based on – emerged in this period. While some of its supporters claim using coloured light for healing dates back to , it’s hard to find evidence of this now.

Modern chromotherapy owes a lot to the fertile mind of physician Edwin Babbitt (1828-1905) from the United States. Babbitt’s 1878 book was based on experiments with coloured light and his own visions and clairvoyant insights. .

Babbitt invented a portable stained-glass window called the , designed to restore the balance of the body’s natural coloured energy. Sitting for set periods under the coloured lights from the window was said to restore your health.

Indian entrepreneur Dinshah Ghadiali (1873-1966) read about this, moved to the US and invented his own instrument, the , in 1920.

The theory behind the Spectro-Chrome was that the human body was made up of four elements – oxygen (blue), hydrogen (red), nitrogen (green) and carbon (yellow). When these colours were , it caused sickness.

Some hour-long sessions with the Spectro-Chrome would . By using its green light, for example, you could reportedly aid your pituitary gland, while yellow light helped your digestion.

By 1946 Ghadiali had made from sales of this device in the US.

And today?

While some of these treatments sound bizarre, we now know certain coloured lights treat some illnesses and disorders.

is used to treat newborn babies with jaundice in hospital. People with seasonal affective disorder (sometimes known as winter depression) can be treated with regular exposure to . And ultraviolet light is used to treat skin conditions, .

Today, light therapy has even found its way into the . LED face masks, with celebrity endorsements, to fight acne and reduce signs of ageing.

But like all forms of light, exposure to it has both risks and benefits. In the case of these LED face masks, they could disrupt your sleep.

This is the final article in our ‘Light and health’ series, where we look at how light affects our physical and mental health in sometimes surprising ways. Read other articles .