Circadian Lighting Design for the Home: Criteria, Standards and Lighting design guidance

Lighting plays a crucial role in regulating our circadian rhythms - the internal 24-hour clock that governs sleep-wake cycles, hormone production, body temperature and other bodily functions. Properly designed circadian lighting in the home can enhance wellbeing, improve sleep quality, and boost alertness and concentration during the day. Here we'll explore some key criteria and standards for circadian lighting design in homes, with practical examples. 

Embracing the Superiority of Natural Light

Natural daylight is considered the optimal source of light for humans due to its high intensity and full spectrum. It supports human vision, psychology, biology and life in general (e.g. photosynthesis, phytomorphology and more). Outdoor light intensities during the day are significantly higher than indoor light, ranging from ten to five hundred times greater.

Here are suggestions to incorporate more natural daylight into our daily lives:

• Begin the day by spending at least 30 minutes outdoors in the morning, immediately after waking up. This can be achieved through activities such as taking an outdoor walk to the metro and bus while commuting to work or school. If it is still dark at the time of waking, aim to experience the first half hour of natural daylight after sunrise.
• Allocate a minimum of 30 minutes during lunchtime to be spent outside in daylight. This can involve taking a walk from or to a restaurant and office or simply enjoying the outdoor environment.
• Exercise or play outdoors during daylight hours.
• Children should spend a minimum of two hours outside in natural daylight each day.
• Sit close to a window (<1 meter) and face the outside. This allows for indirect exposure to natural daylight.

While going out, remember to protect the skin and eyes from excessive UV radiation and stay hydrated.

The Problem of Inadequate Circadian effective Lighting:

Most of us spend 90% of our time indoors, this can affect our biology. Many homes and buildings today are designed without considering the impact of lighting on our circadian rhythms. Standard lighting often provides uniform illumination levels and color temperatures throughout the day, which does not align with our natural circadian cycle. This mismatch can lead to a host of health issues, including:

Sleep Disorders: Exposure to bright, blue-enriched light in the evenings can suppress melatonin production, making it harder to fall asleep and leading to poor sleep quality.

Reduced Alertness and Productivity: Lack of bright light exposure during the day, particularly in the morning, can result in feelings of drowsiness, decreased alertness, and lower productivity.

Mood Disturbances: Insufficient exposure to daytime light has been linked to seasonal affective disorder (SAD), depression, and other mood disorders.

Metabolic and Hormonal Disruption: Circadian rhythm disruption can affect the regulation of hormones like cortisol and insulin, potentially increasing the risk of metabolic disorders and obesity.

Difference Between Normal Lighting and Circadian Lighting:

Electric lighting was originally designed for human vision. E.g. it enables us to see and be seen. The human eye (for photopic vision) has its highest relative sensitivity to the light at around 550nm (green), and has a much lower sensitivity of red and blue. Light sources have been optimized to provide light output at that particular wavelength range and ignoring other wavelengths. This increased the energy efficiency of those light sources and the way we measure light (lumen/ lux) are based on the human vision only. And while it was long believed that light influences our sleep/wake cycle, the principles and scientific facts were not known until the 21^st century when scientist discovered the third receptor (melanopsin) in the human eye that regulates the melatonin release. The 2017 Nobel Prize in Physiology or Medicine was jointly awarded to Jeffrey C. Hall, Michael Rosbash and Michael W. Young for their discoveries of molecular mechanisms controlling the circadian rhythm.

Normal lighting primarily focuses on providing adequate illumination for visual tasks and creating a desired ambiance. It often uses a fixed color temperature and intensity throughout the day. While this may meet basic visual needs, it does not take into account the non-visual effects of light on our circadian system.

Aspect	Normal Lighting	Circadian Lighting
Purpose	Providing adequate illumination for visual tasks and creating a desired ambiance.	Sufficient light for visual tasks and designed to mimic (natural variations in) daylight intensity (and color temperature) to support the internal circadian clock.
Intensity	Often uses a fixed illumination level throughout the day.	Provides bright light (500 lux or more) during the day to promote alertness and dimmer light in the evenings to facilitate relaxation and sleep.
Color Temperature	Usually employs a fixed color temperature throughout the day and can adapt tunable white strategies, although not designed for circadian impact.	Uses cooler color temperatures (4000-6500K) during the day to suppress melatonin and boost alertness, while shifting to warmer color temperatures (2700-3000K) in the evenings to minimize circadian disruption.
Timing and Control	Does not consider the timing of light exposure and its impact on the circadian system.	Emphasizes bright light in the morning and reduces light levels and blue light content in the hours before bedtime. Smart controls and sensors can automate these transitions based on time of day and occupancy.
Melanopic Intensity	Does not take into account the melanopic intensity of light, which represents the relative impact of light on the circadian system.	Considers the melanopic intensity of light and aims to meet standards like the WELL Building Standard, which recommends a minimum melanopic lux of 250 during the day.
Health Impact	It may lead to circadian rhythm disruption, sleep disorders, reduced alertness, mood disturbances, and metabolic issues.	Supports natural circadian rhythms, promoting better sleep, improved daytime alertness, and overall health and well-being.
Energy Efficiency	Focuses on energy efficient visual illumination. Energy efficiency is generally prioritized over excellent quality of light parameters for R9, Glare and human centric light.	Focuses on energy efficient lighting for visual, emotional and its biological role. Quality of light and benefits for people are prioritized over energy efficiency alone.

 

Quality of Light remains important

Quality of light refers to the various characteristics of light that affect visual comfort, performance, and overall well-being in a space. The specific requirement depends on the lighting application. Key aspects of light quality include:

Illuminance (Lux): Measured in lux, illuminance quantifies the amount of light falling on a surface. Adequate illuminance levels are crucial for visual acuity and task performance. Recommended lux levels vary depending on the space type and activity. E.g. 500lux is a common requirement for task lighting. 300lux for regular office desk. Up to 150lux for living spaces.

Uniformity: Uniformity describes the evenness of light distribution across a space. Good uniformity reduces eye strain and enhances visual comfort. It is typically expressed as a ratio of minimum to average illuminance levels.

Glare: Glare is the visual discomfort caused by excessive brightness or contrast. It can be direct (from light sources) or indirect (from reflective surfaces). Minimizing glare through proper luminaire selection, placement, and shielding is essential for visual comfort.

Color Rendering Index (CRI): CRI measures a light source's ability to accurately render colors compared to a reference source. A higher CRI (maximum 100) indicates better color rendering. A CRI of 80 or higher (90) is generally recommended for indoor spaces.

R9 Value: R9 is a specific CRI value that measures the rendering of saturated red colors. A high R9 value (50 or above) is important for accurately rendering red tones, which is crucial in certain applications like healthcare and retail. At entrance level an R9 of 10 is required.

Flicker Frequency: Flicker refers to the rapid variations in light output. High flicker frequencies (above 1000 Hz) are less noticeable and less likely to cause visual discomfort or health issues. Lower flicker frequencies should be avoided.

Energy Efficiency (Watts per m²): Energy efficiency in lighting is often expressed as the power consumption per unit area (watts per square meter). Lower values indicate higher efficiency. Efficient lighting design and the use of LED technology can significantly reduce energy consumption. Over the years this requirement has become more stringent with current requirements around 5 to 8 watts per m² in large spaces.

Optional Mentions:

Daylight Sensors and Presence Sensors: These sensors can automatically adjust artificial lighting based on available daylight and occupancy, enhancing energy efficiency and user comfort.

Verification: Post-installation verification through light level measurements and user feedback helps ensure that the lighting design meets the intended quality criteria and user needs.

By considering these aspects of light quality, designers can create comfortable, visually appealing, and energy-efficient spaces.

Standards and guidelines with relevance for Circadian lighting

Some relevant standards and guidelines include:

WELL Building Standard V2- Light Concept

UL Design Guideline 24480 - Promoting Circadian Entrainment with Light for Day-Active People

ANSI/IES RP-28-16 Lighting and the Visual Environment for Seniors and the Low Vision Population

DIN SPEC 67600 Biologically effective illumination

IALD + LIRC GUIDELINES FOR WELL RATING SYSTEMS

There are different definitions of circadian effective light. The most popular are melanopic lux (or equivalent melanopic lux) which is popular in North America. As well as MEDI, which is popular in Europe.

EML: based on International WELL Building Standard

M-EDI: based on DIN Technical Spec 67600

CS: based on UL Design Guideline 24480 

Example requirements

WELL Building Standard (WELL v2): Light Concept L03: Circadian Lighting Design Requires projects to provide at least 200 equivalent melanopic lux (EML) at 75% or more of workstations, measured on the vertical plane facing forward, 1.2 m [4 ft] above finished floor (to simulate the view of the occupant). This light level should be present for at least 4 hours per day for every day of the year.

It is important to note that circadian effective light should be measured at eye-level. It is about the light that hits the eye. So in offices it should indeed be measured at 1.2meter height. In hospitals and care homes the pillow-height is probably more relevant. Note that it is better to have the brightest intensity synched with the solar noon time.

Singapore's Green Mark 2021 Healthy Buildings Guidance Note:

1. Engage a lighting specialist to look at the various opportunities for a bespoke circadian lighting system that balances brightness and dimming with graduated colour shift during the day. Typically the system would reference either Equivalent Melanopic Lux (EML) and/or Melanopic Equivalent Daylight Illuminance (MEDI). 2. A project can install a dynamic lighting system with colour shift settings that graduate between colour temperatures during the day. At least 3 colour shifts should be considered during the day. A recommended programme is below for a building with normal daytime occupancy. Note that the colour changes should be gradual and recommended to be over a 1hour period. Recommended Colour Temperature Range 7am to 10am 3500-4000K 10am to 2pm 5000K-6500K 2pm to 5pm 3500K-4000K 5pm onwards 2700 -3000K

This guideline does not specify threshold values. It does include some recommendations on CCT. CCT is about the color appearance of white light to the human eye. Higher CCTs are likely to have a higher melanopic lux ratio. However this may not be accurate. Hence it is recommended to design with melanopic lux criteria directly. 

BREEAM (UK New Construction 2018): Hea 01 Visual Comfort - Daylighting Encourages the provision of good levels of daylighting to occupied spaces, promoting circadian rhythms and a connection to the outdoors. Requires a minimum daylight factor for 80% of floor area in occupied spaces or minimum illuminance levels of 300 lux for 2,000 hours per year in occupied spaces.

 

These are just a few examples of the specific criteria related to circadian lighting found in these standards and guidelines. Each standard provides detailed requirements and recommendations for lighting design, control, and assessment to promote occupant health and well-being through supportive circadian lighting strategies.

Circadian light in small flats

In a small flat, the lighting plays a crucial role in creating a comfortable and functional living space that caters to the diverse activities and needs of its residents. 

Adequate general lighting is essential to provide a well-lit environment. This ensures that residents can move around safely and perform daily tasks with ease. Task lighting should provide focused, bright illumination, enhancing visibility and reducing eye strain during activities such as reading, cooking, or working on projects. 

Considering the limited space, efficient use of lighting fixtures is crucial. This means fixtures that provide bright, uniform, low glare lighting can be used to illuminate the space. 

Natural light should not be overlooked. Whenever possible, maximizing natural light sources through the strategic placement of windows or skylights can greatly enhance the overall lighting quality in a small flat. Natural light not only creates a pleasant and inviting atmosphere but also offers numerous health benefits, such as boosting mood, improving productivity, and supporting the body's natural circadian rhythm. 

To address individual preferences and needs, adjustable lighting features can be incorporated.

Considering the diverse lifestyles and schedules of residents, lighting should be flexible and adaptable (e.g. ranging from function light, to task lighting as well as circadian effective lighting)

By carefully considering the lighting applications in small flats and addressing the various human activities and needs, a well-designed lighting system can enhance the functionality, aesthetics, and overall comfort of these living spaces, ultimately improving the quality of life for the residents.

Circadian light specification:

LIGHTING DESIGN

Intensity and Illuminance: Light intensity affects alertness. Brighter light (~500 lux or more and 250 EML) during the day promotes wakefulness, while dimmer light in evenings signals the body to wind down for sleep. Lights should have the ability to provide both bright light and dim light.

Color Temperature and Spectrum: Lights should provide white light at a specific point or along the BBL (black body line) and have an enriched spectrum to be more effective in supporting the circadian clock. It could optionally provide changing CCT where cooler color temperatures (4000-6500K) are ideal for daytime and warmer color temperatures (2200-3000K) are better in evenings. For shift workers light that mimic sunrise and sunset (from 900K) is recommended.

Timing and Duration: Bright light exposure early in the day helps regulate the circadian clock. It should ideally follow the local solar noontime. In the 2-3 hours before bed, light intensity should be lowered (or at least its melanopic content) to avoid sleep disruption.

Glare and Visual Comfort: Lighting should be diffuse and evenly distributed to minimize glare and provide visual comfort. CRI should ideally be 90 with a high R9 if possible.

LIGHTING FIXTURES

Normal lighting:

LedAffinity Ceiling Round Φ500mm: 40W with 4000 lumens output (100 lm/W efficiency), featuring good color rendering (CRI 80) and adjustable color temperature from 3000K to 6500K. AC220-240V input, IP20 rated, white housing, backed by 2-year warranty.

Circadian lighting:

CircadianLux Ceiling Round Φ500mm: 40W with 3487 lumens output (87 lm/W efficiency), featuring superior color rendering (CRI 96, R9 90) and adjustable color temperature from 3000K to 6500K. Melanopic ratio ranges 0.806-1.131. Wide 110° beam angle, AC220-240V input, IP20 rated, white housing, backed by 2-year warranty. This light can satisfy requirements under the WELL standard.