It sounds absurd, but the biological mechanism exists for this phenomenon to occur.
One thing that has always confused the traditional model of eye biology has been the fact that some people with complete blindness can still sense day-night cycles.
While many blind individuals suffer from insomnia, the inability for sustained and regular sleep, some have healthy sleep patterns aligned to daily changes in sunlight.
This suggested that, while unable to consciously perceive the scene of a sunset, for example, their brain is subconsciously aware of the receding light during dusk and prepares the body for sleep.
Cue this 1995 study that provided strong evidence blind individuals could detect light.
Researchers exposed blind patients to bright, artificial light for 90 minutes and then measured melatonin levels in the blood. Melatonin is a hormone that controls sleep cycles, naturally increasing at night to help us sleep. They found that bright light exposure strongly suppressed blood melatonin levels. Incredibly, while the patients did not report being able to see any light, their brains and bodies acted as if the sun had suddenly risen.
This led to a search for cells in the eye that could sense external light-dark cycles but not contribute to typical conscious vision – so called “time-of-day sensors.”
In 2002, multiple teams of researchers converged on a small group of cells in the mouse eye: ipRGCs (intrinsically-photosensitive retinal ganglion cells). These microscopic time-of-day sensors make up only a tiny fraction of total eye tissue, which previously made them elusive to neuroscientists studying the eye.
The newly identified ipRGCs looked familiar to scientists in some ways. Both ipRGCs and traditional vision detectors are neurons – specialized cells of the brain and body that communicate using electrical impulses.
Each type sends thread-like projections spanning from the eye to the brain. But scientists quickly discovered biological properties of ipRGCs that support a unique role in sensing environmental sunlight vs. darkness.
ipRGCs have three basic properties:
They can directly sense light; they are intrinsically photosensitive. They accomplish through a molecular sensor called melanopsin. Melanopsin is found on the surface of ipRGCs, can absorb individual particles of light, and can trigger ipRGCs to send electrical impulses to the brain. Imagine melanopsin as a light switch, and ipRGCs as a wire relaying electrical messages from the eye to the brain. When the switch is flipped on, ipRGCs tell the brain whether it is dim or sunny outside.
Second, ipRGCs have an unusual pattern of connectivity with the brain. Most neurons that originate from the eye are wired to brain centers meant for visual processing. ipRGCs, on the other hand, form connections with over a dozen non-visual brain regions.
One region is the suprachiasmatic nucleus (SCN), a brain center known as the master circadian clock. The SCN is a master regulator of the entire body’s light-dark rhythms, influencing daily changes in behavior. For example, this brain region causes drowsiness by stimulating the release of melatonin into the bloodstream. Since the brain is encased by a thick layer of skull bone and other tissues, the SCN receives little light. It must therefore use electrical signals from ipRGCs to align bodily processes according to a 24-hour clock. In this case, ipRGCs act as cellular messengers, keeping the brain aware of what time it is.
Third, ipRGCs display unusually prolonged patterns of electrical activity. All neurons communicate with one another using electrical impulses. ipRGCs can discharge electrical impulses for extremely long periods of time. While most neurons only show heightened electrical activity for milliseconds, ipRGCs are activated for as long as they are exposed to light, and for several minutes even after a brief flash.
I have spoken about non-visual photoreceptors before, but I’ll likely dive deeper into them so you have a better understanding of why we’re circadian creatures.
And guess what, light sensing proteins aren’t just in the eye.
They’re also in the skin, fat, blood, blood vessels, and everywhere else throughout the body.
Much love,
Zaid