Circadian lighting considerations.

Jonathan Rush

Jonathan Rush, Partner

Assessing the risks of early adoption.

Part four of our five-part series exploring the latest thinking in circadian lighting – in celebration of the International Day of Light on 16 May.

Part one – Understanding our circadian rhythms
Part two – Understanding the colour spectrum
Part three – Exploring circadian lighting

The ethical question posed in part three of this series is a good starting point when considering the risks of launching head-first into proposing sophisticated circadian lighting systems.

Arguably, the ethical line falls between whether we use the colour spectrum of light to ‘augment’ or to ‘support’.


Using light unnaturally to stimulate a physiological response in order to achieve greater productivity or concentration.

Example: very cool spectrum or blue light during the day to saturate the eye with 480nm light and over-stimulate alertness. 


Using reasonable, considered lighting design to support a natural and considered light colour exposure.

Example: lights that go warmer in colour and dim into the evening to support natural tiredness and sleep. 

Personal differences.

One major issue is the ‘one size fits all’ approach. People are different: from our chronotype (how we sleep), to our geography, work patterns, diet, age, and subjective responses to light. We know the eye yellows and receives less blue spectrum light as we age, and that a person of 50-60 years secretes about 35 percent of the melatonin of a 10-year-old. We also know that personal preference and light quality is important in a person’s appreciation of their internal environment. While the science may favour cooler spectrum lighting during the day, it could be argued that preference, and thus perception of illuminated quality, may contradict this.


One key question is illuminance. Modern building standards have consistently tried to lower illuminance levels and allow more creativity in interior environments but research suggests that higher illuminances a more supportive to melanopsin. This is a major concern for energy loads in a modern, low carbon world.


Public Health England has stated that the blue spectrum output of normal LEDs are not a risk to the human eye, but a circadian system would be looking to increase blue spectrum and it has been seen in rats that this can cause retinal damage. Short wavelength blue light hits the retina faster than long wavelength light, and research suggests this could cause oxidative stress in the retina tissue. This intensity of blue light has also caused the American Medical Association to express concern about “potential harmful human and environmental effects”.

In addition, colour mixing between colour sources requires dimming, and dimming often causes flicker.  Many “electronic” drivers for LEDs have high flicker modulation – similar to that of old fluorescent lighting that a generation said gave them headaches! Could a dimming or colour-mixing system cause detrimental flicker in the workplace?

As the above shows, just as there is no simple or scientifically proven solution to enhancing a person’s physiology, there is no one simple solution to circadian lighting design.

In the final part of this series, I ponder the future of human-centred lighting design – and where circadian lighting might play a part.