Human-centric lighting: office light that changes through the day

Light is not only for seeing; it also affects the body's daily rhythm. What human-centric lighting is, how it is set up and what you can realistically expect from it.

Someone who spends the whole day in a windowless room under the same white light can leave for home in the evening with no sense of how the time has passed. Daylight, by contrast, changes colour and intensity from morning to evening, and the human body has adapted to those signals over an immensely long time. Human-centric lighting (HCL) is an approach that tries to reproduce part of that natural variation indoors.

How light affects the body clock

Besides the rods and cones used for vision, the retina contains a third type of light-sensitive cell. These cells respond mainly to short-wavelength, blue-green light and send signals to the part of the brain that governs the daily rhythm. Strong light rich in blue in the morning supports alertness and "sets" the body clock. In the evening, the same kind of light can delay the release of the sleep hormone melatonin.

Ordinary lux values are not enough to assess this effect, because a lux meter measures according to the eye's visual sensitivity. The International Commission on Illumination (CIE) has introduced melanopic quantities for this purpose in its CIE S 026 standard. The practical takeaway is simple: both the spectrum of the light and the amount reaching the eye matter.

Light at the eye, not on the desk

Conventional office design looks at horizontal illuminance on the desk surface. The human-centric approach focuses on vertical illuminance at eye level, because the light that affects the body clock is the light entering the eye. This calls for several changes in the design:

• lighting walls and ceilings, since bright surfaces put more light into the field of view;
• direct-indirect luminaires or panels with wide light distribution;
• light-coloured, matt wall finishes;
• placing workstations near windows to make the most of daylight.

There is a tension here: delivering more light to the eye must not increase glare. That is why large surfaces of low brightness are a better choice than small, bright points.

A scenario through the day

A typical human-centric system is built with luminaires whose colour temperature and brightness can be adjusted (tunable white). An example scenario might look like this:

• Morning: brighter, cooler light to support alertness.
• Midday: neutral light at a normal working level; when daylight is strong, the artificial light dims automatically.
• Afternoon: a short period of more activating light during the post-lunch dip.
• Evening: warmer, dimmer light, especially for those working late.

Specific values vary from source to source and research is still ongoing, so there is no fixed recipe. The most sensible approach is gentle, barely noticeable transitions, and letting staff adjust the light at their own workstation within certain limits.

The technical side

Such a system is more complex than a switch-and-lamp circuit. The luminaires have two channels (warm and cool LEDs), and control usually runs over the DALI protocol — in particular DALI DT8, which supports colour control — or over wireless systems. A central controller runs the scenario according to the time of day, sometimes also using daylight sensors.

When choosing a system, check:

• Is colour rendering maintained as colour temperature changes, and do intermediate settings avoid a greenish or pinkish tint?
• Does flicker appear at low brightness?
• Do all luminaires respond to the same command with the same colour, with no differences between production batches?
• Can users override the scenario temporarily, and does the system return to automatic mode afterwards?

Realistic expectations

Human-centric lighting is not a magic solution. It brings the most benefit in offices with little daylight, in shift-work environments, in care homes and wherever people spend long periods indoors. In an office with large windows and plenty of daylight, the most effective "human-centric" step is often simply moving desks closer to the windows and managing the blinds properly.

Treat firm claims from vendors about productivity or health with caution. A well-designed system can improve comfort, but it does not replace the basic quality requirements: enough light, no glare and no flicker.

Zaferoğlu Elektrik presents the Zəfər İşığı lamps made at its Ganja plant in an online catalogue. For standard office lighting needs, you can filter the products there and request an offer.