Human vision is a masterpiece: we can see in bright sunlight all the way down to dim starlight (this is called photopic-, mesopic-, and scotopic vision). We see colors, shapes, contrasts, movements and perceive a constant and reproducible external world around us. Apart from these complex tasks for vision, our eyes play a crucial role in keeping our circadian rhythms in synch. A specific set of light-sensitive ganglion cells in the retina receive and transmit day- and night-signals to the master clock in the central nervous system, which relay this information to organs and cells in the body,
Before the light signal reaches these retinal cells, it passes through the cornea, the lens and the vitreous body. These structures are important for focusing clearly. The cornea exerts the main refractive power (adjusting the direction of light beams), whereas the fine tuning of refraction and accommodation (the process by which the eye changes optical refraction to maintain a clear image as distance varies) is performed by the lens. These mechanisms ensure that a sharp image is formed in the center of the retina, the so-called fovea centralis, the point of our sharpest vision.
The visual cells in the retina are the primary light receptors containing the photosensitive visual pigments, the rods for dim light vision and sensitivity, the cones for daylight vision, colors, contrasts and movements. Nerve cells within the retina receive and process the primary signals coming from rods and cones, and this information is transmitted via the optic nerve to the brain.
The light sensitive ganglion cells contain a specific visual pigment (melanopsin), and their signals are transmitted to the brain via optic nerve and retinohypothalamic tract fibers that branch out from the optic nerve. Rods and cones for image forming vision, and light sensitive ganglion cells, work in concert. The visual cells broaden the range of illuminance received by the ganglion cells. Furthermore, these ganglion cells contribute to the regulation of pupillary responses. Their responses are sustained, in contrast to the fast responses of rods and cones.
Young eyes and those of the elderly have different properties. Older eyes have a reduced light transparency and the retina is less light sensitive. It means that with increasing age we need more light for tasks like reading or fine manual work. For the circadian clock this implies that light input to the master clock may be reduced. Blind patients and those with low vision use different signals, such as social cues, meal timing, bedtimes, and regular daily activities to help synchronize their rhythms. Nevertheless, many of those patients suffer from rhythm and sleep disturbances.
There is ongoing debate as to whether the blue filter intraocular lens which is often implanted after cataract surgery would reduce the circadian light input, because the blue filter lens reduces violet and blue light transmission. Preliminary findings indicate that a UV-filter lens in patients with previous cataracts may minimize the adverse age-related effects on circadian rhythms, cognition, and sleep compared with blue-blocking lenses. However, further study is needed, particularly since blue–blocking lenses don’t “block,” but reduce, specific components of blue, with contrasting transmission spectra.
Charlotte Remé MD is a professor emeritus at the University of Zürich, an ophthalmologist and cell biologist who has defined many rhythms in the mammalian retina, contributed to the understanding of molecular and cellular mechanisms of retinal degenerations and the effects of light on the eye (positive and negative). She has worked with light therapists and CET to ensure safety standards for the field.