Circadian lighting explained

What is circadian lighting?

Circadian lighting is lighting designed with the body clock in mind, not only our eyesight. Most indoor lighting is planned so that we can see comfortably: enough brightness on the desk, good colour, no glare. Circadian lighting adds a second question. What does this light tell the body about the time of day?

You will also hear it called circadian rhythm lighting, biodynamic lighting or, more broadly, human-centric lighting. The names overlap. The idea underneath them is the same: light is a biological signal as well as a visual one, and indoor lighting can be designed to respect that.

How light sets the body clock

Each of us has an internal clock that runs on a cycle of roughly 24 hours. It helps organise when we tend to feel alert and when we tend to feel sleepy, and it influences many processes in the body that rise and fall across the day and night. This daily pattern is called a circadian rhythm.

The master clock sits in a small region of the brain called the suprachiasmatic nucleus. Its role was established by researchers in the 1970s, including a Harvard Medical School team led by Dr Martin Moore-Ede, now Chief Health Advisor to Sensio BioLabs.

Left alone, the clock drifts a little each day. Light is the strongest cue that keeps it in step with the world outside. Specialised light-sensitive cells in the eye, separate from the ones we use to see, send a timing signal to the master clock. Their discovery around the turn of the century is one of the reasons circadian lighting has become a serious field of study rather than a marketing phrase.

Why most indoor lighting falls short

For most of human history, the pattern was simple: bright daylight by day, darkness at night. Modern life has changed that. A large US study of daily activity found that people spent around 87% of their time inside buildings (Klepeis and colleagues, 2001). Indoors, light levels at the eye are usually far lower than outside, even on a grey day. In the evening, many of us then sit under bright artificial light and screens.

The result is a kind of mismatch: often too little light during the day and too much at night, compared with the conditions the body clock evolved for. Researchers are still working out what that means for people over the long term, but it is the problem circadian lighting sets out to address.

Circadian lighting, human-centric lighting and tunable white

These terms are often used as if they mean the same thing. They do not.

Term	What it usually means
Circadian lighting	Lighting planned around its effect on the body clock, including timing, amount and type of light at the eye.
Human-centric lighting	A wider umbrella covering visual comfort, wellbeing and circadian effects together. Often used in office and building design.
Tunable white	A product feature that shifts light between warmer and cooler white. It can be part of a circadian scheme, but on its own it is not circadian lighting.

We explore the difference in more detail in Human-centric lighting: what buildings get wrong.

What good circadian lighting takes into account

• Timing. The same light can have different effects in the morning and late in the evening.
• The light that reaches the eye. Standard lighting design measures light falling on a desk. The body clock responds to light entering the eye, which is a different measurement.
• The type of light. Not all light is equal as a timing signal. Two lamps that look similar can send quite different signals.
• The person. Age, routine and setting all matter. Published research suggests, for example, that the eye lets less light through as it ages.
• The evening and night. Good design considers when to reduce light, not only when to add it.

How circadian light is measured

Ordinary light measurements, in lux, describe how bright light looks to us. They do not describe how strongly it acts on the body clock. In 2018 the International Commission on Illumination (CIE) published a standard, CIE S 026, that defines measures for these non-visual effects of light. The most widely used is melanopic equivalent daylight illuminance, often shortened to melanopic EDI.

Building standards have started to adopt these measures. The WELL Building Standard, for example, includes a circadian lighting feature based on melanopic light at the eye. For anyone specifying lighting, this is the practical shift: from asking only how bright a room is, to asking what light the people in it actually receive.

What the research does and does not show

The core science is well established. Light is the main signal that sets the human body clock, and its timing and amount both matter. Beyond that, the picture is still developing. Many studies are small or short, results vary between settings, and how best to apply laboratory findings in real homes, workplaces and care settings is an active area of research.

That is why we are cautious about claims. Circadian lighting is not a medical treatment, and nobody should present it as one. What the evidence does support is taking the light around us seriously, measuring it properly and designing it with care.

Where circadian lighting is being used

Interest is growing in places where people spend long hours indoors or keep unusual schedules: hospitals and care settings, workplaces, hotels serving travellers who cross time zones, and increasingly homes. Each setting has different needs, which is one reason a single fixed lighting scheme rarely suits everyone.

Frequently asked questions

Is circadian lighting the same as blue light?

No. Blue-rich light plays a part in the body clock signal, but circadian lighting is about the right light at the right time, which can mean more light in the day and less at night.

Do I need special bulbs?

Not necessarily. Daylight is the strongest natural signal, and time outdoors in the morning is free. Purpose-designed lighting becomes more useful where daylight is limited.

Is circadian lighting a medical treatment?

No. It is a way of designing everyday lighting. If you have concerns about your sleep or health, speak to your GP.

Where Sensio BioLabs fits

Sensio BioLabs is the science arm of Sensio Group. We study how the light people actually receive through the day relates to how they feel and function, and we are developing patent-pending technology that measures that light, delivers what is missing and learns what works for each person. Read more in our Insights, or get in touch.