PART 2. LIGHT: SENSING BEYOND VISIONFeb 10, 2021
Want to know something amazing?
Even with our eyes closed, our bodies can ‘see’ light!
Sunlight is essential for human survival and we’ve developed complex systems for sensing it that extends beyond vision.
HOW THE SKIN “SEES”
We recently discovered nonvisual light-sensing molecules - a class of photoreceptors or “opsins” - distributed in skin and parts of our brains.
While we understand how visual “phototransduction” (light-induced electrochemical signaling) works in the eye, how nonvisual light-sensing works in the skin and other organs is still a mystery.
We now have some clues, largely thanks to evolution:
In 2017, scientists were stunned to find the photoreceptor, melanopsin, not only in the eyes, but in human skin and fat!
The authors showed that these cells were indeed light-sensitive and that exposure to the equivalent of blue light from the sun can activate metabolic, fat-burning processes in them.
We’re now aware of many types of photoreceptors in our eyes and skin, which are our major light-sensing organs.
The pineal gland and other parts of the nervous system also seem equipped for the job based on the photoreceptors they express.
WHY WOULD WE EVOLVE THIS WAY?
This review summarizes the roles of these receptors nicely:
The role of the nonvisual photoreception is to synchronise functions of living organisms to the environmental light periods in order to help the survival of various species in different biotopes [environments]. In vertebrates, the so-called deep brain…photoreceptors, the pineal organs… and the retina… are involved in the light-based entrainment of endogenous circadian clocks present in various organs….In all species examined so far, deep brain photoreceptors play a role in the circadian and circannual regulation [seasonal] regulation of periodic functions.
In other words, these light-sensing molecules help our brains understand whether it is time to sleep or wake up, be active or sedentary when to mate, what season it is, how to regulate our metabolism, energy production, hormones, and pretty much everything else!
LIGHT IS OUR TIME-KEEPER
I think one of the coolest parts of our light sensing system is that the photoreceptors in our skin and eyes detect different wavelengths of visible and ultraviolet light, which informs our bodies about our environment:
Proportions of red, blue and green light that comprise natural light vary according to the time of day and seasons. We evolved to detect and respond to these varying spectral signatures via photoreceptors on our skin and eyes, and that’s a big part of how our bodies know what time of day/year it is.
Signals downstream of these photoreceptors then dramatically change how our cells, organs and the whole human system functions throughout each day, week, month and year (there are many rhythms associated with each).
This includes most body functions: body temperature, sleep cycles, blood circulation, blinking, pulse, hormonal secretions (like growth hormone, insulin, cortisol, sex hormones), heart rate, thermoregulation, urination, bowel activity, nostril dilation, appetite, and arousal, to name a few!
So, in this way, light is one of the key signals controlling the essential rhythms in the human body that drive most human behaviours.
The fact that your skin senses light has many implications for your health, but the most obvious are:
- Beyond sleeping with your eyes covered, it is important to make sure your bedroom is as dark as possible if you sleep with any skin exposed.
- Even when you’re not looking directly at them, your body detects ambient light from screens and bright lights around you, meaning your hypothalamus won’t order the production of ‘sleep hormone’ melatonin, destroying your chances of healthy sleep.
- Along the same lines, blue light detected through the skin also promotes wakefulness and enhances cognitive performance by increasing activity in the brain regions associated with memory, supporting that light at night keeps our brains active.
So you can see why good “light hygiene” is about more than just throwing on a pair of blue-light blocking glasses…
Our elegant light-sensing systems are designed to keep us attuned to the daily rhythm of the sunrise and sunset. When we deviate too far from that natural rhythm by exposing ourselves to artificial light, our systems become dysregulated and we eventually become ill.
POOR LIGHT HYGIENE AFFECTS WAY MORE JUST SLEEP…
Beyond affecting sleep stages, late-night light exposure affects several major circadian systems in the body that can affect how restful a night’s sleep can be, including:
1. Metabolism: Late-night light exposure slows the metabolism and insulin is less able to regulate glucose levels. Blood sugar remains high during sleep, which often elevates heart rate, causes “night sweats”, nightmares and general restlessness. Over time, high blood glucose levels lead to weight gain, and eventually contribute to insulin resistance and ultimately, type 2 diabetes.
We also now know that melatonin levels directly regulate key parts of the hunger/satiety pathway, including the “hunger hormone”, leptin, causing us to overeat when we should have stopped eating for the day. (Melatonin is a hormone produced in the pineal gland in the absence of blue and green light after sunset.)
2. Brain health: New studies are also pointing to the critical role of circadian rhythms in building proteins used for healthy neuron functioning in the brain and how sleep deprivation interferes with this process. (Our bodies have two peak times for brain-supporting protein production: the first is late in the evening when you’re starting to feel drowsy, the other is right before you wake up). Disrupting these is a risk factor for dementia, Alzheimer’s, memory loss, and general cognitive decline.
3. The immune system: We’ve known for many years that adequate rest is critical for the maintenance and proper function of the immune system. When we are sleep-deprived, we get sick more easily.
4. Fertility: All hormones in our bodies are controlled by “clock genes”, whose precisely-timed oscillations are influenced by light (as mentioned in Part I). To have correctly oscillating clock genes and fertile bodies, we must manage light correctly, as well as eat, exercise and sleep at specific times of the day. Alterations or disruptions of biological rhythms are linked to significant disruptions in reproductive function. Altered circadian rhythms lead to altered hormonal secretion patterns, reduced conception rates and increased miscarriage rates.
Most other hormones linked to reproductive health (ie. prolactin) are secreted at higher levels during sleep and is suppressed by inadequate sleep. Estrogen has also been known to modify clock genes and circadian rhythms in the reproductive system in a complex feedback system.
5. Our microbiomes: Fascinating new research reveals a complex interplay between or circadian rhythm and the diurnal rhythm of our all-important gut and skin bacteria. Disrupted circadian rhythms impact them tremendously, in turn, impacting digestion, the immune system, intestinal stem cell cycles, gut healing, skin healing, aging and a variety of human diseases.
…And so on.
There are too many other detrimental effects of poor light hygiene to list here, so I’ll leave it there. Suffice to say:
Good light hygiene and a healthy circadian rhythm are the cornerstones of good health.
In the next article, we’ll get to “Light Hygiene 101” & how to harness the power of light as medicine.