Circadian Lighting

Circadian rhythms

US scientists have developed a special tool to help designers create lighting installations which affect the sleep-wake cycle.

Using a free calculator, lighting professionals can specify the right light sources and light levels to stimulate the so-called ‘circadian’ rhythm of building occupants.

The first step in establishing whether a lighting system will deliver a prescribed amount of circadian stimulus (CS) is to determine the spectral irradiance distribution of the light incident at the cornea.From this spectral irradiance distribution it is then possible to calculate CS, which is the effectiveness of the spectrally weighted irradiance at the cornea from threshold (CS = 0.1) to saturation (CS = 0.7).Exposure to a CS of 0.3 or greater at the eye, for at least one hour in the early part of the day, is effective for stimulating the circadian system and is associated with better sleep and improved behaviour and mood. To use the calculator, designers should formulate a base condition by evaluating the space using the CS calculator and commercially available lighting design software such as AGi32.The design can later be fine-tuned by again using the CS calculator, while also accommodating IES or SLL recommendations, energy codes, and any client workspace specifications.

The researchers – based at the Lighting Research Center at Rensselaer Polytechnic Institute in New York state – have worked out the type of lighting needed to influence the body’s biochemistry.

Dubbed the circadian stimulus – or CS – metric, this is the type and intensity of the lighting needed to suppress the hormones that make us feel sleepy.

‘Lighting for the circadian system employs lighting design objectives that differ from those typically used in traditional architectural lighting design, and therefore, requires metrics that differ from those currently used by lighting designers,’ Professor Mariana Figueiro, Light and Health Program Director at the LRC, told Lux.

The CS metric is based on the scientists’ model of how the retina converts light signals into neural signals for the circadian system. Specifically, it identifies the lighting needed to suppress the sleep hormone melatonin.

When specifying lighting for the circadian system, the LRC says it’s important to consider light level, spectral composition, timing and duration of the exposure, and the previous light exposures of the occupants.

We have recently upgraded our spectrometer to reveal on light test the CS metric as well as the Human Photopigment calculator

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