day and night, and it is essential for good health, wellbeing, and restorative sleep, that the influence of artificial light works with this rhythm, and not against it. One of the most important aspects of this hormonal system is the regulation of melatonin, which signals when to sleep and when to wake up. In addition to a circadian rhythm, melatonin levels also have a seasonal or circannual rhythm, with higher levels in the autumn and winter when there is less daylight, and lower levels in the spring and summer.

If the rhythm is not maintained, this can have an adverse effect on health. The processes regulated by the circadian rhythm are also responsible for regulating many bodily functions that revolve around the 24-hour cycle, such as temperature, heart rate, and blood pressure. When light to dark (day to night) patterns become asynchronous with a person’s activity and rest patterns, as can occur when we travel across time zones or for night shift workers, circadian disruption or misalignment may occur.

When morning comes and light first hits our eyes, our blood pressure and body temperature begin to rise, our heart rate increases, and there is a reduction in the release of melatonin, which helps us to wake up. Our activity levels increase during the day before decreasing at night, as our body functions slow down and melatonin levels begin to rise again to help us sleep. Most people’s body clocks are set for sleep to begin at about 11 pm, and to wake at about 7 am. Most people are sleepiest from 2-4 am, and from 2-3 pm, and are most alert in the early morning and late afternoon. However, some people’s circadian rhythms differ, and their bodies respond differently at times that are irregular to others, while others work or have schedules that fall outside normal times to meet the growing demands of a 24-hour society. Conflict can result when an individual’s circadian rhythms do not sync with everyone else’s.

So, what happens when an individual’s circadian rhythm is disrupted by external forces or circumstances, or their biological clock is ‘out of sync’ with those of others? They begin to develop circadian rhythm sleep disorders, which, if left unchecked, can lead to issues ranging from exhaustion and confusion to obesity, diabetes, depression, and dementia.

New evidence has shown that the entrainment of the circadian rhythm through the light to dark cycles, which occur from day to night, takes place not in the rods or cones of the retina, but in the intrinsically photosensitive retinal ganglion cells (ipRGCs) of the eye. The functions of the circadian rhythm take place in the part of the brain known as the hypothalamus, within which are a group of cells known as the suprachiasmatic nucleus (SCN), which are connected to the optic nerves that sense changes in light via the ipRGCs. Therefore, the eye has at least two separate pathways for receiving signals

30 The benefits of human-centric lighting technology.

Table 1. The EML of a light source indicates how well it can affect the circadian rhythm.

CCT (K) Light source EML ratio 2700 LED

3000 Fluorescent 4000 LED

6500 Fluorescent 6500 Daylight

0.45 0.45 0.76 1.02 1.10

from a light incident on the retina/optic nerve: the visual response – rods and cones – photopic (and scotopic) lux, ie day and night vision; and the non-visual response – melanopsin ipRGCs – melanopic light (EML), which is used to regulate the circadian rhythm.

Figure 2 shows how we see during the day (the rods and cones in the retina are called daytime or photopic vision), which peaks in the green (555 nm), and reduces as we move into red and blue. As we switch to night vision, the rods mostly take over, and our eyes become more efficient at seeing colours towards the blue end of the spectrum – dark adapted or scotopic vision. The circadian rhythm is most strongly influenced by the blue light detected by ipRGCs in the retina. The biological effects of light can be

Non- Visual

NightLight Visual

DayLight Visual

measured in equivalent melanopic lux (EML) that is weighted to the ipRGCs instead of to the cones (it is the cones that are simulated with traditional lux measurements). Therefore, the EML of a light source indicates how well it can affect the circadian rhythm (see Table 1). So, in general, the higher the CCT, the higher the EML, and the more influence the light source will have on the circadian rhythm. While this is a simplistic analogy, it does stack up if you remember that the higher the CCT, the more blue light (ie the hotter the object), and that the iPRGCs in the retina are more attuned to the blue wavelengths.

Artificial light

Human beings have evolved to be able to live in an outdoor environment modulated by sunlight during the day and darkness at night. Over the past 150 years, we have increasingly relied on artificial light to enable us to perform tasks indoors, and therefore our natural circadian rhythm has been disrupted. Inadequate lighting during the day, and too much light at night, confuses the circadian rhythm, and leads to poor sleep, poor concentration, and reduced wellbeing.

Figure 2. How we see during the day and at night.

As we get older, a number of changes in normal vision occur that are simply due to the ageing process, which exacerbate the issue. As many older people spend most of their time indoors, these problems are likely to be exacerbated. Key changes include: the need for additional light due to a reduction in the efficacy of the ageing eye; the amount of blue light that reaches the retina in particular reduces with age, thus worsening the circadian rhythm response; an increased sensitivity to glare; reduced peripheral vision; reduced sensitivity to contrast; slower adaptation to changes in light levels; reduced visual acuity; reduced perception of depth, and altered perception of colour, ie being less able to discriminate between unsaturated (less intense) colours such as pastels. What the eye sees, the brain must interpret, and this can vary in different circumstances and in those with conditions such as dementia and other conditions. Understanding that an older person will probably see their surroundings differently is a good starting point when designing living environments – including lighting


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