Circadian Lighting: Bringing Nature Indoors to improve our Health and Well-being

Light is more than just illumination; it's a fundamental regulator of our biology. It profoundly influences our circadian rhythms, the internal 24-hour clock that orchestrates sleep-wake cycles, hormone production, and various bodily functions. For homeowners, building owners and professionals, understanding and implementing circadian lighting design can unlock significant improvements in well-being, sleep quality, and daytime alertness.

Written by Jille Kuipers

The Crucial Role of Natural Light

Natural daylight is the optimal light source for humans due to its high intensity and full spectrum, supporting vision, psychology, biology, and general life processes. Outdoor light intensities are remarkably higher than indoor light, often ranging from ten to five hundred times greater. This significant difference highlights why we need to actively incorporate more natural light into our daily lives.

Here are some practical suggestions:

Morning Exposure: Spend at least 30 minutes outdoors immediately after waking, even if it's still dark, aiming for the first half-hour of natural daylight after sunrise. This could involve a walk during your commute.

Lunchtime Outdoors: Allocate a minimum of 30 minutes during lunchtime to be spent outside.

Outdoor Activities: Exercise or play outdoors during daylight hours.

Window Proximity: Sit close to a window (less than 1 meter) and face outwards for indirect exposure to natural light.

Children's Exposure: Children should spend a minimum of two hours outside in natural daylight daily.

The Earth's rotation and its daily light-dark cycle primarily drive our circadian rhythm. Without this regular 24-hour light-dark cycle, our internal body clock, which averages about 24.2 hours, would run autonomously. This slight discrepancy can lead to our bodies feeling sleepy during the day and awake at night, similar to jetlag, resulting in fatigue, headaches, and reduced performance and well-being.

The Science Behind Circadian Light and Hormones

Light's most apparent effect is enabling vision. However, the right type of lighting at the right time also positively influences health, well-being, alertness, and sleep.

Around the year 2000, a "new" third photoreceptor, called melanopsin, was discovered in a cell type within the eye's retina. Melanopsin regulates the biological effects of light and influences the production of hormones crucial for a healthy sleep/waking pattern, known as the "circadian rhythm". The hormones cortisol (making us alert) and melatonin (making us sleepy) are key players in regulating activity and rest. Light signals enter the eye and are transmitted via the retinohypothalamic tract to the suprachiasmatic nucleus (SCN) in the brain, which controls circadian rhythms, and also to the pineal gland, which produces melatonin (Figure 1).

Beyond Visual Lux: Understanding Equivalent Melanopic Lux (EML) and Melanopic Equivalent Daylight Illumination (M-EDI) 

While traditional lux measurements are optimized for the human eye's visible light response (peaking at 550nm), the spectral sensitivity for circadian system activation differs, peaking around 490nm. To quantify light relevant for circadian support, metrics like Equivalent Melanopic Lux (EML) or Melanopic EDI (MEDI) are used. Some manufacturers, like CircadianLux, report the melanopic lux ratio of their lighting products, as these specialized wavelengths can be mapped.

A sunny day outdoors can provide 100,000 lux, while a cloudy day offers 2,000 lux. In contrast, indoor light levels are generally much lower, often a quarter or less of outdoor levels. For instance, a typical office desk might have 300-500 lux for task lighting. This significant reduction in light intensity indoors can negatively impact our circadian rhythm and energy levels. Research shows that appropriate light levels, spectrum, uniformity, position, and timing can significantly improve well-being, productivity, sleep, mood, concentration, and alertness. Studies have even found that office workers experienced higher alertness and performance, less evening fatigue, and improved sleep quality with lights that had more biologically efficient wavelengths.

Key Principles of Circadian Lighting Design

Circadian lighting systems prioritize supporting human circadian rhythms, unlike typical tunable white lights that are optimized for general illumination. This means that conventional tunable lighting generally has a lower EML output compared to lighting specifically designed for circadian support, which focuses on spectral composition and melanopic response.

The EML formula is a crucial tool for quantifying the biological (melanopic) effect of lighting on humans. It emphasizes the response of the retinal ganglion cells (ipRPGs) rather than the cones, which is the focus of traditional lux measurements. Vertical lux, which measures light shining vertically and entering the eye at eye level, is a key consideration in circadian lighting design.

RELATIVE SENSITIVITY TO LIGHT OF PHOTOPIC VISION AND CIRCADIAN LIGHT RECEPTORS

In Nature, 1 Lux equals 1 Melanopic Lux, providing equal support for visual function and circadian function.

In general LEDs 1 Lux equals on average 0.8 Melanopic Lux, which supports visual function but are substandard for circadian function.

CircadianLux LEDs supports visual function and circadian function.

Criteria for effective circadian lighting design

Here are key criteria for effective circadian lighting design:

Intensity and Illuminance:

Brighter light (around 500 lux or more and 250 EML) during the day promotes wakefulness, while dimmer light in the evenings signals the body to prepare for sleep. Lighting systems should offer both bright and dim light capabilities.

Note that it is important to measure the intensity on eye-height level. e.g. in an office context, approximately at 1.20meters (4ft) height measured at the vertical plane. And e.g. in a bedroom context about 0.9meter height with a slightly slanted position.

Color Temperature and Spectrum:

Lights should provide white light with an enriched spectrum for effective circadian support. Cooler color temperatures (4000-6500K) are ideal for daytime, while warmer temperatures (2200-3000K) are better for evenings. For shift workers, light mimicking sunrise and sunset (from 900K) is recommended.

Timing and Duration:

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

Glare and Visual Comfort:

Lighting should be diffuse and evenly distributed to minimize glare and ensure visual comfort. A Color Rendering Index (CRI) of 90, with a high R9 value, is ideal.

Practical Applications of Circadian Lighting

When implementing circadian-effective light, the measurement point is critical. It should be taken at eye-level to capture the light biologically relevant for circadian rhythms. In offices, this means 1.2 meters high to approximate average seated eye-level, while in healthcare settings, pillow height may be more appropriate for bed-bound patients. Aligning peak intensity with solar noon supports natural circadian entrainment.

Example Scenarios for Circadian supportive spaces:

Living Space:

Provide at least 200 EML at 75% or more of tables, measured on the vertical plane facing forward, 1.2 meters above the floor. This light level should be present for at least 4 hours per day (e.g., 10 am to 2 pm). A vertical illuminance of 270 lux with the CircadianLux spectrum can yield 250 EML, which is within the recommended range of the Well Building Standard.

Bedroom

Wake Up: Gradually increase bright light with 125 EML to 200 EML over at least 15 minutes before the target wake-up time. Light color can transition from approximately 900K to 4000K or be fixed at warm or neutral white. This helps people wake up naturally.

Night Time: For toilet breaks, provide visible light with a maximum of 10 lux or 10 EML, preferably below 2 lux or 2 EML. Nightlights should have a low melanopic lux ratio to allow for natural melatonin production.

Activity based circadian lighting

Activity Mode:

Provide at least 225 EML in 80% of the occupied space for at least 6 hours per day. Color temperature should be in the 3500K to 6500K range.

Performance Mode:

Similar to Activity Mode, with CCT between 4000K and 3500K and high CRI.

Immersive Multi-Media Mode: 

Lower brightness requirements, with CCT moving between 3000K and 2700K.

The Future of Lighting for Health

CircadianLux offers various products, including ceiling lights, panels, linear lights, and floor lights, with options for fixed or flexible output and control via on/off or automatic schedules based on geolocation. These lights can save up to 50% energy compared to fluorescent lighting.

Beyond lighting, air quality also plays a significant role in well-being. The ROOMYOU Sensor monitors both CO2 and circadian supportive light (EML), making it a valuable tool for creating supportive and productive environments. Elevated indoor CO2 levels can impair cognitive function, with decision-making abilities decreasing by 15% at 1,000 ppm and cognitive function dropping by up to 50% at 1,400 ppm. Above 2,000 ppm, significant drowsiness and potential health issues can occur.

The integration of circadian lighting and air quality monitoring is becoming increasingly important for promoting health and well-being in various settings, including schools and care facilities for older people. For example, in the Netherlands, Room You and CircadianLux lights help schools meet the Class A requirements of the Fresh Schools Standard, the highest rating available.

As we spend more time indoors, especially older individuals who can spend up to 95% of their time inside, the need for lighting solutions that support our natural biological rhythms is paramount. Circadian lighting systems hold immense potential to improve sleep, cognitive function, mood, and overall quality of life for everyone, from children in schools to older residents in care facilities, and even the staff who work there. This is truly "better light for a better human species".

Circadianlux lights are currently available in Hong Kong, Singapore, The Netherlands and Italy. For other regions, please inquire for availability at info@luxbalance.com