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Human Centric v Circadian Lighting

Human centric lighting (HCL) is lighting that enhances human experience, performance, health or wellbeing. It goes beyond performing the narrow function of enabling us to see better.

Circadian lighting is designed to act upon our circadian rhythm, either reinforcing it or subtly exploiting it for a beneficial end. Circadian lighting, or lighting that uses circadian principles, should be seen as one component of human centric lighting.


For too long lighting design, especially in the workplace, has focussed on lux levels, colour rendering indices (CRI) and unified glare ratings (UGR). Standards such as EN12464 and lighting design guides published by CIBSE recommended minimum lux levels and CRI and maximum UGRs for different types of task. The (unintended) consequence of this has been a tick-box approach to lighting design. While standards were met and guidelines adhered to the result was dull lighting, under-performing workers and students and, in the worst cases, absenteeism, headaches and depression.

Human centric lighting goes beyond merely enabling us to see well. It takes account of the non-visual aspects of lighting, so a lighting designer setting-out to create a human centric installation would also consider:

  • User control. Allowing individuals, or small groups, to have some control over the lighting in their area.
  • Daylight integration. Exploiting daylight so that the qualities of natural light, and its diurnal variation, become apparent to the occupants of a building.
  • Circadian principles. Our body clock and hormone balance are influenced by different wavelengths of light at different times in the day. Circadian principles in lighting design take account of this by enabling the colour temperature and intensity of light to be adjusted through the day.
  • Intensity. In many lighting schemes, uniformity is a goal. However, varying light levels across a space can help to sustain visual interest.
  • Direction. An excess of overhead light can be both boring and give rise to unwanted shadowing, especially of faces. A human centric approach would typically seek to provide some diversity of directionality.
  • Flicker. This is a known cause of distraction and headaches. HCL would eliminate flicker.


Circadian lighting is lighting that delivers changing wavelengths and intensities of light to either reinforce our bodies’ natural daily waking and sleeping cycle or subtly influence it to achieve a particular behavioural result.

Background. In a natural environment the colour temperature and intensity of the light round us changes from dawn, through midday to dusk. For example, the sunlight at noon is rich in shorter (blue) wavelengths which supresses melatonin production, thus keeping us alert. As more of the longer (red) wavelengths are produced towards dusk our melatonin output starts to rise and our bodies slow down and prepare to sleep. To our bodies these daily changes in natural light act like a clock, triggering the production of different hormones that in turn cause fluctuations of our body temperature, our appetite, our wakefulness and our mood. This daily cycle is known as our circadian rhythm.

The problem. Traditional lighting emits the same intensity and quality of light regardless of the time of day or night. The effect of this on people living and working under artificial light for long hours is to reduce or eliminate the stimuli that keep our circadian rhythm strong and healthy. When this happens, the balance of hormones, such as melatonin, and neurotransmitters, such as serotonin and dopamine, is disrupted resulting in depression, anxiety, drowsiness, an inability to get a good night’s sleep, loss of appetite and more.

A solution. Circadian lighting sets out to replicate the natural changes in light that take place from dawn, through midday to dusk, helping us to maintain a healthy hormone balance, mood and sleep pattern. Subtle exploitation of circadian principles, such as a boost of bright light with a raised blue content to combat drowsiness after lunch, can lead to improvements in classroom performance and learning.


Implementing circadian lighting requires several elements:

  • Colour tuneable and dimmable light fittings. These are light fittings whose output can be adjusted through the day to give a changing mix of wavelengths (colour temperature) and overall light output (dimming). To do this requires several arrays of LEDs in each light fitting, each array having its own spectral distribution. In addition, suitable drivers are required that can manage the output of each array separately, altering the overall colour temperature and overall output as required. Typically, these will be “DALI device type 8” drivers.
  • A control system is required to generate the necessary commands for the drivers. DALI-2 is the most widely used lighting control system protocol and it contains a set of commands for managing colour temperature and light output. (For more  details, please read our article What is DALI ? ) Many lighting control systems are available that use DALI-2 and can therefore be used as part of a circadian lighting implementation. For DALI-based lighting controls available from NVC, please check NVC DALI Networked Lighting Controls - Wired and NVC Mesh - Networked Controls - Wireless . Both of these systems can be used in a circadian lighting implementation.
  • Design & commissioning. Circadian lighting does not follow a rigid pattern. Circadian principles can be implemented differently according to the usage of the space being lit and the cultural context. For example:
    • In a school, circadian principles could be exploited by using lower light levels and more of the longer (red) wavelengths after the mid-morning break to help children calm down after a spell in the playground.
    • In an office in China, after lunch lower light levels and warm hues (more red and yellow) would help employees to take a 30-minute nap, which is the norm in China. At the end of the rest period a boost of brighter, cooler (more blue) light would help get them back to work. Conversely….
    • In an office in the UK, after lunch higher light levels and a boost of short (blue) wavelengths would help people stay alert after lunch. This might be a healthier option than strong coffee!
    • At the start of the night shift in a call centre, when elsewhere everyone is slowing down and going home a boost of bright and shorter (blue) wavelengths would help counteract the naturally drowsy feeling of the night shift workers as they arrive.


Circadian lighting stimulates the intrinsically photosensitive retinal ganglion cells (ipRGCs) in our eyes to interact with the suprachiasmatic nucleus (SCN) in our brain which controls our daily waking and sleeping rhythm.

Let’s unpack that.

Most of us are familiar with rods and cones – the cells in the retina responsible for our vision. Rods are sensitive in low-light conditions and give us a black and white view of the world. Cones are effective in brighter light conditions and are responsible for our colour vision. Since 1923 scientists have been aware that mammals are sensitive to light even if they have no rods or cones, so there must, they deduced, be other photoreceptors in the eye even if they don’t provide what we would normally call vision. This third group of photoreceptors are the ipRGCs.

In 2002 and 2003 the role of the ipRGCs and how they functioned were finally both discovered. In short, ipRGCs contain melanopsin, which reacts to light. More than that, it reacts differently depending on the wavelength of light, and its reaction is especially strong in blue light. The ipRGCs are connected to the SCN and the SCN controls the production of melatonin and other hormones and neurotransmitters.

Circadian lighting acts through the ipRGCs on the SCN, stimulating physiological responses as the light intensity and wavelengths vary. At one, entirely benign, level artificial circadian lighting is merely replacing the natural stimulation that we lose by spending many hours indoors and is thereby contributing to our wellbeing. On the other hand, circadian principles could be exploited to produce, at least in the short term, specific outcomes such as increased productivity during night shifts or resilience in the face of jet lag.

The interaction of the ipRGCs, the SCN and the pineal gland (amongst others) is extremely complex and varies somewhat from one person to another. After all, some of us perform best in the morning (larks) and some of us come to life much later (owls). Here is a brief summary of just some of what is going on in this complex interaction.

MELATONIN This is a hormone that promotes sleep. During the day there is very little melatonin circulating in our bloodstream, but the level increases towards the end of the day, causing us to want to sleep. Blue light, especially in short bursts during the day, can arrest melatonin production and can therefore combat drowsiness.

ADENOSINE is a metabolite – something left over after a metabolic reaction. Adenosine tri-phosphate (ATP) transfers energy between cells in our body and when the ATP (and the energy it carries) has been used, adenosine is what is left. Adenosine interacts with various neural receptors causing a decrease in neural activity – drowsiness. This interaction can be temporarily halted by bright light, preventing the onset of drowsiness. Generally, adenosine levels start low and gradually increase through the day. During sleep the body gets rid of adenosine, preparing us for the day ahead.

SEROTONIN and DOPAMINE are both neurotransmitters - they carry signals between nerve cells. They have a complex relationship with each other and with the pineal gland where melatonin (the sleep hormone) is produced. Serotonin production is boosted by exposure to light (so we accumulate more as the day wears on) but in turn it helps the production of melatonin (which causes us to become sleepy towards the end of the day). Dopamine is released when we have a pleasurable experience and it increases our alertness by supressing melatonin production.