Lighting Design for the Circadian Clock

Some people bemoan the onset of dark winter days because during the waking hours they experience lethargy, low motivation, loss of alert response and even depression. The common acronym for this condition is SAD, seasonal affective disorder.

These symptoms may also be experienced by people who work night shift or a day job in a dark environment.

Usually, some level of daily exposure to “full spectrum” light has been proposed as the cure for SAD.

The name “seasonal affective disorder” first appeared in the 1970’s. After many years of research, the name, the SAD acronym and the prescription for full spectrum light only partially reflect our current understanding of the complex interaction of light and human well being.

The Lighting Research Center at Rennsellaer Polytechnic Institute, provides a current definition of a possible underlying cause of what we have often termed SAD: “Biological rhythms that repeat approximately every 24 hours are called circadian rhythms. Light is the main stimulus that helps the circadian clock, and thus circadian rhythms, keep a synchronized rhythm with the 24-hour day. If lack of synchrony or circadian disruption occurs, we may experience decrements in physiological functions, neurobehavioral performance, and sleep.”

The wake / sleep cycle is partically driven by a hormone called melatonin made by the pineal gland. The levels of melatonin in the body are influenced by blue spectrum light exposure to the human eye. Under normal conditions, when melatonin levels in the body are high because of minimal or no exposure to blue wavelength light, we get drowsy and eventually we sleep. When melatonin levels are low, influenced by exposure to high levels of blue spectrum light, we are awake and alert.

For some people “circadian disruptions” occur in winter when it’s still dark outside an hour or two after awakening and when the interior lighting is characterized by warm color temperatures. For night shift workers (eg. medical staff) who sleep during the daylight hours and work under electric lights at night, this same circadian disruption can occur.

Basic Principles of the Circadian System

The following principles can help in the selection of circadian system appropriate lighting for different types of spaces.

  • Light, specifically light with a preponderance of blue wavelengths, triggers the suppression of melatonin – a circadian stimulus.
  • Daylight is the reference point for light with a strong blue wavelength component. Light from fluorescent and LED light sources with a color temperature of 5000K or higher also has a significant blue wavelength component and is usually considered a good circadian stimulus.
  • The intensity, or light level, of blue spectrum dominant light is also important for circadian stimulus. A high output source of 3500K light can be an effective circadian stimulus.
  • For the most precise understanding of the blue wavelength component of a light source, the spectral power distribution, SPD curve should be evaluated.
  • When considering the lighting design for a space, visual acuity parameters (the right amount of light, low glare, good color rendering, etc) may be different than the lighting requirements for optimizing the circadian clock (color temperature, light intensity). Where there is conflict, visual acuity takes priority and other strategies need to be used (see Example 1 below) to address the circadian clock.

Two Examples of Lighting Designed to Address the Circadian System

Example 1

The workspace for night shift hospital staff may be illuminated primarily by 3500K T8 fluorescent lights. This moderate level of blue spectrum light accompanied by a moderate intensity of light may not be enough to keep some staff feeling at peak performance.

One solution used by medical centers has been to have a break room illuminated by high intensity, 5000K or 6500K fluorescent lights. Intermittent use of this room with exposure to strong levels of blue spectrum light helps maintain melatonin suppression throughout working hours.




Example 2

With LED luminaries, the option of tunable color temperature and light intensity becomes a possibility. For assisted living type facilities this approach provides healthy living benefits for the residents. Early morning, high light level 5000K illumination for a circadian stimulus is maintained until noon, when the LED luminaries automatically move to 3500K until 6pm and finally to lower light level 3000K for the evening hours.



Designers and facility managers need to be clear about the lighting design objectives for the many types of unique spaces they encounter. With increased emphasis on healthy buildings, circadian system principles should to be considered along with traditional visual acuity lighting specifications.

For more detail on this topic download a white paper entitled Circadian Stimulus published by the Light Research Center.

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Dave Burtner

Dave has been active in the lighting industry since 1994. Formerly a member of the Illuminating Engineering Society of North America and certified by the National Council On Qualifications for Lighting Professionals, Dave now writes blog posts, lighting tips and provides lighting product assistance for the Topbulb website.

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