‘Bright light disrupts circadian rhythm’
At a time when abundant luminosity is synonymous with economic progress, a growing body of scientific research links excessive artificial lights to ecological chaos and illnesses among humans. Shanthakumar Wilson Rajaratnam, professor of sleep and circadian medicine at Monash University in Australia, speaks to Down To Earth on how light pollution impacts the circadian rhythm of species and makes them more susceptible to diseases.
Q: All species have an internal biological clock that has evolved according to the cycle of natural light and natural darkness at night. Does our biological clock get affected by artificial light?
The central biological clock in the brain has evolved to be a self-sustaining clock. That means, it has the capacity to generate and maintain circadian rhythms. Now, we know that it is critically important for these circadian rhythms to be in sync or aligned with the solar light-dark cycle.
Throughout the course of evolution, the solar light-dark cycle has been the major external regulator of our circadian rhythms. It is relatively recently that we have been exposed to artificial light and developed a response to it. This exposure, particularly at certain times of the day, can disrupt circadian rhythms in a number of ways, by shifting their timing and dampening their oscillation. Artificial light exposure can also lead to increased arousal or alertness that can influence our sleep-wake behaviour. There is considerable evidence now that exposure to artificial light has resulted in an increased frequency of such disrupted circadian rhythm.
Q: What implications can a disrupted circadian rhythm have on the human health?
As I said, we as organisms have evolved to be aligned with the solar light-dark cycle. There was a time when we would prepare to sleep at dusk and wake up at dawn. But now, because of artificial indoor light, midnight has become the start of the night for most people, instead of the middle of the night. Our circadian rhythms shift to these late hours. This is the first impact that comes with the exposure to artificial light at night.
The second effect it can have is suppression of the secretion of the hormone melatonin. This hormone has antioxidant properties and is known to have a number of important physiological functions, one of which is to help to regulate our sleep. So if melatonin secretion is suppressed due to disrupted circadian rhythms, our sleep can be adversely affected.
In humans, the reduced or lack of exposure to darkness is a risk factor for a number of health conditions. If we disrupt our sleep cycle and circadian rhythms, we expose ourselves to increased risk of metabolic disorders like diabetes, cardiovascular disorders, obesity and a number of mental health conditions. Other risks include productive disturbances, certain forms of cancer, and disruption to the immune system. Not to mention, the impairment to our performance and our productivity. This is because, like the brain, other systems of our body also have circadian clocks. The brain clock plays the important role in keeping synchrony all through the body, which means a disruption here exposes people to increased risk of a number of adverse health conditions.
In countries like India, where cardiometabolic disorders, cardiovascular disease and diabetes have a high prevalence, I wonder whether, as a population health strategy, we should look at preventative measures to try and control light exposure and improve the quality and duration of sleep to reduce the risk factors of these major, well-documented conditions.
Q: It’s clear that dark skies are critical to human health. Can we say, then, that light pollution impinges on one’s rights to natural darkness?
Human right is access to a necessity that helps us function optimally. So I would certainly advocate for light exposure at night to be controlled and regulated. The ability to sleep and exist with optimally timed circadian rhythms should be part of a person's basic rights and privileges.
Q: Now that artificial light is part of our lives, do you think that species will eventually evolve to adapt to this night-time luminosity?
Several academic publications are now coming out on the benefits of natural light exposure during the daytime on physical and mental health, and on reducing evening light exposure to try and optimise our circadian health. All of these practices have been well documented in a number of ancient cultures. I think it is time to re-examine those and return to the deep knowledge that has been held in ancient societies and practices, because we will probably get from them answers to the myriad health challenges we are seeing.
With regard to the future, we will eventually adapt. There are a number of genes that regulate the circadian clock and light sensitivity. Over time, some people with certain kinds of genes may thrive more in a 24-hour society. But my preference would be to see light pollution being brought under control, rather than having us adapt to it.