Importance of Light in Regulation of Circadian Rhythms

As discussed in my previous article, circadian rhythms for our physiological system endogenously (within the organism) follow a just over 24 hour rhythm, about 24.2 hours on average[1]. Without something to realign them, the rhythms will gradually, day by day, fall more and more out of sync with the solar 24-hour day. Something must signal to our body when the 24-hour day restarts. Consequently, our circadian system has mechanisms to respond to external cues (called zeitgebers) from our environment.

There are many zeitgebers – even social interaction can have an effect the timing of the circadian clock – but the three most important are timing of exercise[*], eating[?], and light exposure. Of these three external cues, light is recognized as the most important external cue to align the body to the solar 24-hour day. Evolutionarily, it makes sense that light would be the primary driver to align circadian rhythms to the day. Humans evolved in equatorial Africa with 12-hour natural cycles of daylight and night. While food may be scarce and daily activity levels fluctuate, the one thing that was and is constant is the rise and fall of the sun. This inherent predictability makes the day – light cycle a very stable external cue to ensure the circadian system is aligned to the 24-hour day[?].

That light affected the circadian rhythm of the SCN was understood since the at least the 1970’s[10], it wasn’t until the discovery of the intrinsically photosensitive retinal ganglion cell (ipRGC) around 2001 that its direct connection was confirmed. I’ve already reflected on the significance of that discovery in my first article (a ganglion cell that responds to light!) so I’ll spare you that here, but it’s worth taking a moment to get into the weeds about how the ipRGC responds to light.

How the ipRGC Responds to Light

As the name implies, the ipRGC is intrinsically photosensitive, and are also called the melanopsin receptor because it uses the melanopsin protein to respond to light. The ipRGC has some characteristics that make it fundamentally different from our visual system’s rods and cones[11].??

The implications for these differences are profound for how we design light for human health. Whereas the visual system responds instantly to create our visual experience, the melanopsin system is slow and requires high light levels to truly activate. The ipRGC responds to light allows it to sense the passage of time and how bright it is, responding to a kind of stable bright light evolved to track the course of daylight and the shift to night. For me, it begins to explain why it’s a collection of light over time that is so important for light and health, and why duration of light exposure is just as important, and in fact intertwined with, the timing and amount of light exposure in determining our circadian system’s response to light. But I’m getting ahead of myself, and we’ll come back to that in another article.

The Blue – Yellow Opponency Circuit

While ipRGC’s directly respond to light, they also take inputs from our cone visual receptors via the blue-yellow (b-y) opponency circuitry[14]. I confess, as a non-scientist, fully understanding this is challenging, and I think that is one of the reasons why it’s largely ignored in the broader discussion of circadian lighting. Essentially, we have three types of cone receptors that respond to different wavelengths of light, resulting in our color vision (broadly speaking red, green, and blue spectrums). The response of the red and green cones combine in the retinal circuitry as the “yellow” response, while the blue cone is the “blue” response. The circuitry combines the responses from “yellow” and “blue” and they oppose each other, muting the response of one over the other. This input goes into the ipRGC and is packaged together with its own intrinsic photoresponse[12] and the signal is sent off to the brain.

The net result is that the functional response of the ipRGC and our circadian system to light is far more dynamic and complicated than is appreciated in the broader lighting and architectural design community. I will discuss this in more depth in a future article.

Light, the SCN, and the Cortisol-Melatonin Cycle

Light, and its opposite, darkness, directly signals the SCN via the ipRGC (indeed, the ipRGC is necessary for signaling the SCN), and both light and darkness are equally important for healthy entrainment of our circadian rhythms. Bright light exposure in the morning suppresses melatonin and signals the SCN to align the peripheral clocks to the day through hormonal, nervous system, and body temperature signals. A key component of this is the release of cortisol, an important hormone commonly demonized for its role in the stress response, but necessary in the morning to wake us up from sleep. The SCN determines the timing for the adrenal gland to release cortisol and other glucocorticoids[15,16], signaling the body to wake up and prepare for action and the day.

At night, the absence of light signals the SCN to release melatonin via the pineal gland. The functional opposite of cortisol, melatonin signals the rest of the body to shift to sleep and repair mode and a key component in a healthy sleep-wake cycle. Melatonin is often mistaken as a hormone that puts you to sleep, but it does not: under natural light conditions, melatonin levels begin to rise 2-3 hours before sleep onset[17]. It is simply the signal for the body shift to rest and repair mode. Other factors such as core body temperature and relaxation actually induce the drift off to sleep[17]. Melatonin also has potent anti-oxidant affects[18,19], offering a natural protection from cancers, and it is hypothesized that reduced melatonin production from light exposure and circadian disruption could contribute to the development of certain cancers associated with shift work[20].

Taken together, the light-regulated circadian rhythm of the SCN and resulting daily cycle of cortisol-melatonin drive a healthy sleep-wake cycle. The sleep-wake cycle is perhaps the most obvious and most important circadian rhythm, since sleep is foundationally important to health[17]. Indeed, the two are so inextricably linked it can be difficult to parse between circadian disruption and sleep disruption. We will explore the importance of sleep, how light impacts sleep, and how improving light can improve health, performance, and productivity through improved sleep quality next.

[*] Timing of exercise appears to affect the cardiovascular system’s[2] and muscle peripheral clocks[3] and does affect the SCN upstream. Late exercise can cause a phase delay at the SCN and disrupt the sleep-wake cycle[4]. I experience that regularly from my jiu jitsu practice, where evening sparring sessions inevitably reduce my sleep quality despite my increased discipline with light and timing for eating. On the other hand, exercising outside in the morning is my favorite way to create a powerful signal to my circadian system using both light and activity.

[?] Timing of eating appears to have a particularly strong effect on the circadian rhythm of the metabolic and digestive systems[5,6] but maybe not the SCN[5,7] and is an interesting thread of study for maintaining metabolic health in shift workers[8].

[?] All mammals have an SCN and use light to entrain their central clock, and even birds, which evolved from dinosaurs, have a kind of SCN that uses light for entraining the circadian clock[9]. It seems, to me, that the dependence on light for entraining the circadian clock is ancient evolutionarily speaking, which says something of how dependent all animal life is to light exposure.

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This article is proudly researched and written by myself without the use of AI.

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