What Types of Light Affect Circadian Rhythms?
All light within the spectrum of visible light has some effect on our circadian rhythms; however, some colours, brightness’s and directions have more of an effect than others.
In 1800, most people across the world worked outside and were exposed to the change from day to night (BBC 2019). Therefore, their circadian rhythms were kept in sync - waking up in the morning with the sunrise, feeling active and alert, then slowly winding down as the day progresses towards sunset. Today, however, is very different. Most people work indoors and have very little exposure to natural daylight. A study conducted by USA Today surveyed around 16,000 people across 14 countries in North America and Europe and found that around 90% of people spend close to 22 hours inside every day!
The colour and intensity of natural light changes throughout the day, beginning with a warm amber light at sunrise, slowly changing into a cooler, brighter light during the day, before warming back into an amber light at sunset.
The eye contains cells that react to these changes; it sends a signal to an area of the brain called the hypothalamus; this triggers the release of serotonin (the body’s natural antidepressant) during the day and melatonin (a hormone affecting sleep) at night. This creates our circadian rhythm and promotes our mental and physical health, mood, and energy.
Below is a diagram showing the various colours within the spectrum of visible light:
All of these colours serve a purpose; they are present in natural daylight and therefore should be present in artificial light as humans spend increasing amounts of time indoors. Currently, most artificial lighting is a static colour, brightness, direction and wavelength; it either looks like a warm or a cold light, neither of which is healthy when a building occupant is exposed to it all day.
The development of circadian lighting aimed to change this by providing (as close as possible) the same benefits as natural light, regulating human circadian cycles and our sleep and wake patterns. To understand the benefits of circadian lighting in a live environment, watch our Heanor Park care home case study video.
Now, let’s take a look at the effect of colour:
Red light
Wavelengths above 700nm (perceived as yellow/red)of sufficiently high intensity and duration can entrain circadian rhythms primarily through the non-visual neural pathway.
Red light is perfect for evenings because it has a low colour temperature; you can be immersed in red light at night without disrupting your circadian rhythm. If you’re having trouble sleeping and surrounding yourself with unnatural blue light every night, e.g. cold white light bulbs, light from digital devices, etc., that’s likely a big factor.
Yellow light
Yellow and orange light has little effect on our body clock, so that you can use a very dim yellow or orange light at night. However, a yellow light all day wouldn’t be beneficial. We could feel very lethargic, meaning we are less productive and alert. It is important to have a mixture of colour temperatures, the same as natural light. The natural light cycle reveals many variations; these can now be mimicked by circadian lighting to help ensure our circadian rhythms are kept in sync.
Blue light
A cold light will emit more blue wavelengths and have a greater potential disruptive effect on our circadian rhythms if we are exposed to it at the wrong times of the day.
Blue light (wavelengths from 460 to 480nm) is very beneficial in the mid-late morning to wake us up and during the daytime to make us feel alert. It has even been linked to people making fewer mistakes and improved wellbeing. It is the overexposure to blue light that begins to cause issues. It is particularly disruptive at night when it induces the strongest melatonin suppression.
Blue light can be problematic because it has a short wavelength that affects levels of melatonin more than any other wavelength does. Light from fluorescent bulbs and LED lights can produce the same effect. Normally, the pineal gland in the brain begins to release melatonin a couple of hours before bedtime, and melatonin reaches its peak in the middle of the night. When people read on a blue light-emitting device (like a tablet, rather than from a printed book) in the evening, it takes them longer to fall asleep; plus, they tend to have less REM sleep (when dreams occur) and wake up feeling sleepier— even after eight hours of shuteye (National Sleep Foundation N/A).
However, small changes can be made to avoid the amount of blue light exposure in the evening. You can adjust your laptop/computer screen brightness and colour so that the colour moves away from blue to more of a reddish/yellow tone after a certain time. Also, using warmer coloured bulbs in bathrooms and bedrooms helps to minimise the impact; as mentioned above, red light has a higher wavelength; therefore, it does not suppress melatonin release.
Read more about the effect of blue light on our circadian rhythm.
Purple light
Purple light (420nm wavelength) is significantly weaker than the 460nm of blue light, therefore in terms of melatonin suppression, replacing the blue part (460–480nm) of the spectrum with shorter wavelengths during the transition to evening could constitute an alternative in order to maintain synchronisation whilst preserving visual chromaticity (Bonmati-Carrion et al. 2017).
Green light
Green light has a similar effect to blue light; however, green is slightly less powerful. Studies suggest that green and blue light effectively suppress melatonin, but later during the light exposure, the spectral sensitivity to green light was exponentially reduced relative to blue light (Leena Tähkämö et al., 2019).
Blue, in contrast to green light exposure in the evening, shortened REM sleep duration and suppressed slow-wave sleep in the first hours of sleep, with a rebound toward the early morning hours (Leena Tähkämö et al. 2019).
An example of colour variation in Heanor Park care home.
Therefore, there is a very subtle balance to be stricken between the varying colours, intensity and directions of light. This complicated process needs to be completed effectively in order to ensure the building occupant receives the maximum benefit.
Our ‘Circadian Plus’ solution is entitled ‘Plus’ because it delivers lighting that goes beyond just reproducing a (Western European) daylight simulation, it uses the control of spectrum to manage people's Circadian cycle. Our sophisticated algorithm powers the solution meaning that there is no need to a building manager to intervene - the lighting is always emitting the most beneficial combination of each time of day.