The circadian conundrum

Circadian rhythms used to be largely governed by the rising and setting of the sun. The human body evolved to be exposed to strong outdoor light in the daytime, and people usually went to sleep soon after it grew dark. In 1879, when the light bulb was invented by Thomas Edison, it heralded profound changes in how society operated. Artificial lighting extended the length of the evening, in turn reducing the average hours of nightly sleep in industrialised countries.

Different forms of artificial light vary in colour temperature, measured in units of Kelvin (K), that rates a light source on the lighting spectrum ranging between red and blue. “Warm white” yellowish lighting is often between 2700 and 3000K, and bluer “cool white” lighting is likely to be between 4000 and 4500K. Warm white globes tend to be more aesthetically pleasing, and have a cosy feel, whereas cool white is colder and more clinical. While exposure to indoor lighting and other forms of artificial light is hard to avoid, measures can be taken to minimise its health impacts and effects on natural cycles.

In workplaces, where many people spend large chunks of time, lighting may not be optimal. If the opportunity