Light & Circadian Rhythm

During a 24-hour day, exposure to light and darkness in your environment plays a major role in regulating your sleep/wake cycles, body temperature, blood pressure, hormone release – essentially all the moving cogs and pieces of your body clock. In a world of electric light, television, smart phones and built environments, here’s a guide for understanding your circadian rhythm while optimizing your lighting to better emulate the natural environment.

Light & Circadian Rhythm

At the core of circadian rhythm is your relationship with light, which acts as the maestro of many meticulously orchestrated biological processes. To better align your circadian rhythm, understanding lighting design specific to each system is critical.

But before we get into the specifics of electric light and applications in the built environment (below), let’s unpack some key facts about human circadian rhythm in the provided infographic. Incredibly complex and extensively studied, this is just the tip of the iceberg – but it does serve as a primer for understanding the basics of this critical field of study for builders, lighting experts, architects, and designers to name a few.

Circadian Rhythm Infographic
The Secrets of Lighting & Circadian Rhythm

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In Review: The Basics of Circadian Rhythm

To better align your circadian rhythm to natural 24-hour cycles caused by the rotation of the earth, it all starts with knowing the key components – including light itself, melanopsin, the suprachiasmatic nucleus, and melatonin.


The Maestro

At the core of circadian rhythm is light, which acts as the maestro of many meticulously orchestrated biological processes. The pineal gland is stopped from producing melatonin when light-induced activation of the SCN occurs, creating alertness.


The Messenger

A photopigment receptor protein found in retinal ganglion cells in the eye, melanopsin acts as a messenger, sending signals to the suprachiasmatic nucleus when exposed to light.


The Pacemaker

Explained in greater detail below, the suprachiasmatic nucleus (SCN) of the hypothalamus houses the circadian pacemaker, which receives light/dark signals from the eye’s retinal ganglion cells.


The Sandman

Darkness promotes production of melatonin in the pineal gland, causing sleepiness. While melatonin plays many roles, its secretion (or lack thereof) is critical to sleep-wake cycles.

Human-Centric Lighting

For lighting professionals, this term is not new – but it’s changing each year. Human-Centric Lighting (HCL) draws a line between light and wellbeing with a foot planted firmly in verified science. Beyond the health benefits that HCL could yield, it has opened up market opportunities for designers, lighting companies, and of course, consumers. Adding light color temperature to the mix, many smart technologies are becoming more integrated in homes to adjust light intensity and temperature to better suite natural cycles at appropriate times of day.

Biologically Effective Lighting

At its core, biologically effective lighting helps to improve cognitive performance. As LEDs Magazine points out, a link has even been found between high-CCT lighting and enhanced alertness. A 2015 article points to several studies with eye-opening insights that are hard to ignore:

  • A 200 person office implemented HCL luminaries. While annual electric costs rose from approximately $7,790 to $10,000, productivity rose 1.15%, which is equivalent to two additional output hours each month. A 1% decrease in sick days was also reported.
  • When HCL was installed, a school with 1,000 students and 80 teachers increased annual energy costs from approximately $8,900 to $12,240. However, a 15% improvement in cognitive performance was noted in some students while 5.3% of students suffering from ADHD benefited from a 10% reduction in education and healthcare costs.
Emotionally Effective Lighting

Published in Proceeding of the National Academy of Sciences of the United States of America (PNAS), “Spectral quality of light modulates emotional brain responses in humans” touches on the value of emotionally effective lighting in view of seasonal affective disorder:

Light therapy is the treatment of choice for seasonal affective disorder (SAD) and is a promising treatment for other major affective disorders, suggesting that light can modulate mood in the long term.


Tying back to key components of circadian rhythm, “a polymorphism in the melanopsin gene has recently been associated with SAD,” according to the report. Blue-enriched light is equally effective as (visually brighter) white light as an effective treatment of SAD.

Circadian Light Systems

In recent years, the understanding of the physiological mechanism of circadian entrainment has grown significantly. It is rooted in the non-visual physiological responses to light. The visual system is most sensitive to green wavelengths while the non-visual (circadian) system is most sensitive to blue wavelengths.

-US Green Building Council (

Since most of us work in buildings, perhaps tucked far away from a window, electric lighting combined with color shifting components are one proposed solution for lack of access to natural light for still supporting circadian entrainment.

Adding to the mix, Ed Clark and Marty Brennan wrote a piece in METROPOLIS outlining “6 Principles for Designing Spaces That Support Circadian Health“. Without paraphrasing all six of their points – and encouraging you to give their piece a read yourself – here are two points they made about light specifically in view of circadian rhythm:

Blue Light, Green Light
Circadian Sensitivity
Tunable Electric Lights

Cones are particularly sensitive to green light while ganglion cells reach peak sensitivity with blue light. With alerting effects, blue light helps to regulate the body clock. Thus, exposure to blue light should be throttled back before bedtime.

Tunable fixtures, or those that can change color, subtly and gradually change the light source throughout the day and can add variable aesthetics to the physiological and functional aspects of lighting solutions.

Suprachiasmatic Nucleus (SCN) & Resetting Signals

Near-24-Hour Period

The circadian system in animals and humans, being near but not exactly 24-hours in cycle length, must be reset on a daily basis in order to remain in synchrony with external environmental time.

-US National Library of Medicine National Institutes of Health (NCBI)

According to NCBI.NLM.NIH.GOV, circadian rhythms need to be synchronized (or entrained) to the 24-hour day regularly. Natural light/dark exposure is usually enough for syncing to the environment.

The good news is the clock is fickle, and can be course-corrected. The bad news is it can be set off course if the natural light dark cycle is disturbed.

Considering the world of electric light we live in, this presents modern challenges unprecedented in human evolution when you consider the whole of our time on earth as a species before electric light.

More on the "Pacemaker"

Studies published in the early 1970’s established the suprachiasmatic nucleus of the hypothalamus as the central circadian pacemaker in mammals. This pacemaker is comprised of individual cells which, when isolated, can oscillate independently with a near-24-hour period.

In the same report linked here, it’s further detailed how the suprachiasmatic nucleus (SCN) takes info from the retina (via a photopigment called melanopsin), causing entrainment to natural light-dark cycles. The SCN is sometimes referred to as the “master clock” as well.

Since this cycle can adjust based on exposure to light, unnatural light during the biological night can make a big impact on our ability to entrain to your natural environment in a healthy, normal way.

If you aren’t currently considering that hour you spend with your smart phone inches from your face just before bed (those of us who are guilty know who we are), it might be the time to do so!

Beyond the Pacemaker? After groundbreaking studies in the 1970s that established the SCN as the master clock of circadian rhythms, new studies have shown other cells and tissues in the body might be “capable of modulating their activity on a circadian basis,” according to NCBI. Experts don’t believe this diminishes or demotes the SCN from being our central pacemaker, however.

Ebb & Flow | Rhythm-Resetting Signals

A rhythm-resetting signal occurs when continuous darkness is interrupted (e.g. light exposure in otherwise continuous darkness), either shifting the rhythm backward or forward, depending on when the signal was interrupted.

Phase Delay or Advance

At 24.3 hours, the human clock is slightly longer than the 24 hour daily cycle. The proper light stimulus received in morning hours will reduce the 24.3 hour cycle while light exposure in the evening can advance the cycle.


Phase Delay
Phase Delay

Picture an animal just laid down to rest. It’s still early in the night. Light is suddenly introduced into the environment and a phase delay is the result.

Phase Advance
Phase Advance

Picture an animal that has slept through most of the night. Light is suddenly introduced into the environment and a phase advance is the result.

Circadian Rhythm Disorders

Despite the fact that at least 40 million Americans report having sleep problems, more than 60 percent of adults have never been asked about the quality of their sleep by a physician, and fewer than 20 percent ever initiated a discussion about it.

In the simplest terms, circadian rhythm disorders are manifested when the timing of sleep doesn’t match the natural 24 hour sleep/week cycle. Some are advanced while others are delayed. Some are variable while others are quite rare. In all cases, they can cause myriad of issues not limited to fatigue and finding it difficult to “sync” with the rhythms of the wider world.



  • Characterized by abnormally late bed times and subsequent late rise times, reports the incidence of DPS is undetermined in the overall population.
  • More common among teens and young adults, between 7-16% of this population has DPS.
  • It’s also estimated that DPS is found in 10% of people suffering from chronic insomnia.


  • Advanced Sleep Phase Disorder, sometimes called Advanced Sleep-Wake Phase, is characterized by earlier bedtimes roughly between 8-9pm and early rise times between 4 to 5am.
  • Most common among older adults, puts the prevalence of ASPD at just 1%.



  • Shift workers, or those who work the graveyard shift can work in many industries including hospitals, trucking, law enforcement, factory work and more.
  • According to, shift workers make up 3.2% of the workforce.


  • Characterized by travel across one or more time zones, jet lag is a symptom of disrupting the 24-hour cycle.
  • reports that nearly 93% of all travelers will experience jet lag at some point in their travels.

In recent decades, knowledge on the subject of circadian rhythm has exploded, providing new insights about how to live in sync with the environment. Hopefully this guide has helped you untangle just some of circadian rhythms’ many mysteries!

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