Our sleep cycle, from the production of our melatonin to our morning alterness, is goverened by light. And there's one group of little-known cells to thank for regulating the flow of light and keeping your circadian rhythm running properly: Retinal Ganglion Cells (RGCs). Never heard of 'em? You're not alone. Our understanding of the many different kinds of RGCs and what they do is in its infancy, but neurobiologists are beginning to grasp how a certain, tiny subtype are crucial to the daily recalibration of our circadian rhythms and, therefore, crucial to sleep health. Here's what you should know about the microscopic gatekeepers of sleep.
RGCs are found in the inner layer of the retina, that thin neural tissue in the back of the eye. There are thought to be around two-dozen different types (some say more) of them. Their purpose? Put simply, they transmit visual information from the eye to the brain.
Most RGCs receive visual signals from rods and cones, or photoreceptors, which are nerve cells that respond to light. But scientists are learning more about how a very small group of photoreceptor RGCs often called intrinsically photosensitive retinal ganglion cells (ipRGCs). As their name suggests, these are sensitive to light. Unlike rods and cones, which transmit visual cue, they don’t actually allow us to see. Instead, they act as a sort of trigger for when the sun is up, telling our bodies to hold off on the melatonin so we can be awake and alert because, hey, it’s the daytime.
How do they do this? Well the ipRGCs contain the protein and photoreceptor melanopsin and transmits ambient light messages via the retinohypothalamic tract to the suprachiasmatic nucleus, a group of nerve cells in the hypothalamus known as "The Body's Master Clock" that regulates sleep-wake cycles as well as body temperature and hormonal activity. (Now, melanopsin responds to light even without the presence of rods and cones, which was thought to be impossible when rods and cones were the only known light receptors in the retina. But this explains why circadian rhythms aren’t necessarily disrupted in the blind: ambient light transmitted from ipRGCs still reaches to the suprachiasmatic nucleus, even though other visual cues don’t.)
So, after light hits the suprachiasmatic nucleus, ipRGC light signals travel via the superior cervical ganglion to the pineal gland, which produces melatonin, that oh-so-important sleep horomone. Melatonin slowly enters our body throughout the day as light wanes, which is why we're accustomed to feeling tired at night. It's also why we feel the effects of jet lag — skipping time zones affects the schedule by which our body absorbs light.
The modern world, however, is making this system struggle. Light is, quite literally, everywhere, be it from a smart phone screen or blinking billboard, and sometimes the cells don't know how to distinguish one type of light from another.
“Because of the three-to-four billion years of evolution of light during the day and darkness at night, our body clocks are not expecting light at night,” Randy Nelson, Ph.D., the chair of neuroscience at Ohio State University’s Wexner Medical Center, previously told Van Winkle's.
To make matters worse, ipRGCs cells don’t actually treat all light the same way. While red and orange lights can hit them without causing much of an impact, the cells are uniquely sensitive to blue light. That's why that lightwave has become public enemy number one when it comes to sleep — when the cells receive the signals from screens, they perceive the same as sunlight, commanding our bodies to be awake. Hence the introduction of "Night Shift," blue light filtering glasses and other such ways to ward off blue light.
IpRGCs have been integral in helping scientists get more of a grasp on sleep in general. Manipulating the expression of melanopsin at will, as researchers did in a study published in Nature Neuroscience in 2008, could help scientists figure out how to manually regulate the sleep-wake cycle. The study, which exposed rats in deep sleep to short bursts of blue light and found that it resulted in lasting wakefulness, suggests that ipRGCs might play a large part in how alert we are.
This has implications for future treatment that might help swing shift workers, those whose jobs put them in opposition with light's natural schedule, avoid some of the negative health effects that result from their schedules, as well as minimize the effects of jet lag and help defeat Seasonal Affective Disorder.