From childhood, we learn the colours of the electromagnetic spectrum that are visible to the human eye—red, orange, yellow, green, blue, indigo and violet—but the spectrum extends much wider than this. Below red are infrared light, microwaves and radio waves, and above violet are ultraviolet light, X-rays and gamma rays—all of which are undetectable to the human eye. Ultraviolet radiation is produced by high-temperature surfaces like the sun and is the cause of sunburn and skin cancers, but we also depend on it. It stimulates the production of vitamin D in the skin, which is essential for bone health and a healthy immune system—and a lack of it, especially during growth, puts us at risk of weak muscles, inadequate bone mineralization, skeletal deformities, and mineral loss. However, vitamin D is only produced in the skin between certain wavelengths of UV light: wavelengths that are almost never reached within the arctic circles. In the sunless Lovozero, in Russia’s north-west, young children are periodically exposed to UV radiation to prevent stunted growth, creating an imaginary summer—if only for a minute or two.