Circadian Lighting – How Do We Deal With It?

In the latter part of 2017, three scientists, Hall, Rosbash and Young, received a Nobel Prize in Physiology or Medicine for “their discoveries of molecular mechanisms controlling circadian rhythm“. This is the biological clock which controls all animals, including humans, and directs them when to wake, eat and sleep over a 24 hour period.

Many scientists believe that prolonged disruption to a human’s circadian rhythm, such as working night shifts, can cause weight gain, tiredness and depression and may increase, in the long term, the risk of serious health problems, such as heart disease, cancer and diabetes.

Light And The Circadian Rhythm

Non-visual receptors in the human eye known as Retinal Ganglion Receptors (ipRGC) are very photosensitive, and are especially sensitive to blue spectral light or short wavelengths, i.e. those that are found in natural daylight. There is a wealth of research to show that because these receptors are so sensitive, especially to blue light (their peak sensitivity is at a wavelength of about 450-480nm), that they send the brain a ‘wake-up’ signal. This signal is believed to halt the release of the hormone Melatonin (the ‘sleep’ hormone).

This sensitivity blue light explains why we wake up in the morning as blue radiation is at its strongest then. It decreases during the day and is absent at night when our bodies slow down and need to sleep.

LEDs – Problem or Solution?

Research into how light can affect human wake/sleep pattern, known as circadian health, has recently become very interesting to LED technology as it can easily deliver precise colour spectrums, unlike conventional lighting.

It has been shown that ordinary indoor lighting has a significant impact on the circadian cycle which can delay the onset of sleep. This concern has been compounded by the use of LED lighting, which uses blue-pump LEDs, thus causing blue radiation to be an issue even in so-called ‘warm white‘ luminaires. Also, using tablets, phones and watching the tv is another problem. Many tablets and phones now have a blue light reduction facility at night.

There is also evidence that red light causes an alert reaction. This is contra to the perceived wisdom that blue-rich light is a wake-up signal that drives alertness.

Tuning Spectra for the Circadian Cycle

Circadian entrainment, or alignment is proportional to the total dose of blue light. To influence the circadian cycle, one may therefore alter the light level and the spectrum.

LED technology makes it possible to shape a spectrum as desired, and in particular to reorganize the spectrum to add or remove blue radiation, but this isn‘t the end of it. Colour changing LEDs have long had the capacity to add or remove blue light throughout the day. If one simply starts from a white light source and removes all blue radiation the resulting light goes yellow, that is pleasant in a domestic or restaurant evening setting but of no use in a hospital where functional white light is needed.

What Colour of Light is Needed and When?

The current belief is to that to increase the CCT to 4000-5000K in the morning and increase light intensity to 1000 lx and beyond will have a beneficial effect on circadian entrainment. Various medical studies have confirmed that blue-rich, high-CCT light could help synchronize the sleep schedule, in particular for populations whose circadian cycle is otherwise altered by a medical condition.

However, it is much more challenging to create a source with low circadian stimulation in the evening. To reduce the total blue dose, both the amount of light and the spectrum must be adapted, but this ends up as yellow light. This effect is made even worse by the basic operation of the ‘white’ LED. It is in fact a blue LED with a phosphorous coating that makes it look white, a technique called blue-pump.

Getting Rid Of The Blue

Several manufacturers have developed LEDs with violet-pump LEDs that have a reduced output in the blue spectrum.


These LEDs have successfully achieved natural white with much reduced blue light. The downside is a slight hit on efficiency, and as the technology is new, a much higher die price. However, if you need bright light in the evening without too much circadian disruption, these may be of use so we will be offering them as options in our downlights, floor lights and striplights.

Example of a standard floor light, now with a circadian-friendly option


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