CIRCADIAN LIGHTING


awarded the Nobel Prize in Physiology or Medicine for their work isolating the genetic controls of the circadian rhythm and identifying their functions. Research into the genetic functionality


that controls the circadian rhythm is ongoing (though this is beyond the scope of this article). Instead, let us turn to how light and the circadian rhythm interact.


ipRGCs Since the discovery of intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) at the dawn of the 21st century, scientific understanding of how light interacts with and helps to synchronise the human circadian rhythm has advanced significantly. ipRGCs are non-visual photoreceptors


in the eye, which express the photopigment melanopsin in reaction to sunlight. They are a third class of photoreceptor cells in the retina, alongside the more well-known rod and cone photoreceptors responsible for processing visual stimuli. Despite comprising just 1-3 per cent of


photoreceptor cells, research has shown ipRGCs are the most significant conduit between light and the entrainment of the circadian rhythm, as well as triggering bodily responses such as pupil dilation. Melanopsin released by the ipRGCs stimulates the suprachiasmatic nucleus, a tiny structure within the hypothalamus that synchronises the ‘clocks’ of our bodily functions via thousands of nerve cells.


The influence of daylight For tens of thousands of years, human beings evolved in direct sunlight. Days would begin at sunrise and, by necessity, wind down at sunset. We spent ninety per cent of our lives outdoors. These are the circumstances for which our circadian triggers evolved. ipRGCs are mainly located in the lower and nasal portions of the retina, positioned to receive stimulation from a bright, planar source above the head – in other words, sunlight. Some scientists have argued for the


recognition of light as an essential form of nutrition for our bodies. But, like food, it must be the right kind of light at the right time to provide what our bodies need. Sunlight has characteristics which change throughout the day; intensity and colour temperature have a definite cycle, which our circadian rhythm has evolved to follow. As the Sun ascends to its midday peak, the light spectrum is powerful with blue light at a wavelength of 480 nanometres, triggering the ipRGCs in our eyes to produce melanopsin, increasing the production of alertness hormones like dopamine and serotonin and suppressing the sleep hormone melatonin. As the day progresses into late afternoon and evening, the light shifts to oranges and reds and the production of melanopsin


IFHE DIGEST 2025


declines, reversing the balance of sleep and wake hormones.


Electric lighting and its discontents Humans have evolved to interact with daylight. However, our way of life has changed since the Industrial Revolution and the invention of the electric light source. We now spend 90 per cent of our time indoors, often under electric lighting. It is not hyperbole to suggest that the electric light has enabled the modern world – we all work, socialise and conduct our lives beyond the constraints of daylight. This rapid change has affected our bodies, though. Traditional electric light gives an


unvarying light level and colour temperature in contrast to the changing cycle of sunlight. Furthermore, the incandescent lightbulb is almost entirely deficient in the blue end of the colour spectrum, which, as we have noted, is essential for the entrainment of the circadian rhythm. This disruption unbalances hormone production, and this can impact human health. Research has linked circadian disruption


to a wide range of issues, from mood swings, poor mental health and lethargy to Alzheimer’s Disease, infertility and even cancer. To underline the latter, the International Agency for Research on Cancer has declared that working night shifts should be considered a possible carcinogen.1


The LED age and human-centric lighting The widespread switch to Light-Emitting Diodes has revolutionised the lighting industry in recent decades. LED offers many advantages over older technologies: the performance is improved, they are more energy efficient, they last much longer, and over their lifespan, they are cheaper, too. But most pertinently, they are


controllable – the colour temperature, measured in kelvin, is tuneable and can be adjusted over time. This capacity means LEDs can provide lighting of different intensities and spectral profiles, creating dynamic lighting that is sympathetic to the circadian rhythm and helps to entrain it. Tuneable LEDs are the foundation for human-centric lighting. Professor Peter Boyce of the Rensselaer


Polytechnic Institute defines human- centric lighting as “…lighting that considers both the visual and non-visual effects of


Research has linked circadian disruption to a wide range of issues


exposing humans to light and that widens the range of possible effects from visual performance and comfort to sleep quality, alertness, mood and behaviour with consequences for human health.”2 In the lighting industry, human-centric


lighting (referred to by some practitioners as circadian or integrative lighting) is still a discipline in development. Indeed, there are presently no agreed standards, methods or measurements for human- centric lighting. However, industry bodies are engaged; for example, the UK’s Society of Light and Lighting has announced it is preparing a new position statement on the topic. It is to be expected that, as experience and innovations coalesce, agreed best practices and standards will also emerge and be formalised.


Human-centric lighting in the healthcare sector In the healthcare sector, dynamic lighting may benefit both patients and staff. Tuneable lighting is already in use in several intriguing applications. Furthermore, studies to establish further uses have led to some startling results. The following are a handful of recent examples. Cincinnati Children’s Hospital has


recently developed a circadian lighting system specifically for the needs of premature babies, whose connection to the maternal body clock has been severed early. From the assumption that “Spectral lighting could potentially help babies establish healthy wake/sleep cycles, regulate metabolism, promote optimal eye and brain development and maintain a healthy body temperature,” a custom lighting system was developed for the new Neonatal Intensive Care Unit (NICU), which will both help babies now and enable further research to improve the developmental prospects of premature infants.3 The scheme uses indirect and reflected


lighting to illuminate the space, minimising glare to ensure patient comfort. The tuneable LED luminaires used for the project provide a full range of visible light wavelengths programmed to change composition during the daylight cycle. The system can even adjust the length of the lit day to reflect seasonal variations. Dynamic, human-centric lighting also


has potential applications to improve life quality for older people. For example, experiments in care homes have concluded that adjustable lighting providing intense blue-spectrum light in the day with gentler, redder illumination in the evening and at night can significantly reduce the rate of trips and falls among residents. One study recorded a 43 per cent drop in fall injuries versus comparable control sites.4 Yet another application being


investigated is in care facilities for people with dementia and related degenerative


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