How the eye tells time without rods and cones
The eye has two jobs that use light. One is image-forming vision, handled mainly by rods and cones. The other is non-image-forming vision: detecting overall light levels to set the body's daily clock, suppress melatonin, and control the pupil [3]. The key cells for this second job are intrinsically photosensitive retinal ganglion cells, or ipRGCs. They are ganglion cells—the cells whose fibers form the optic nerve—but they also contain melanopsin, a pigment that makes them respond to light directly [3][4].
This arrangement means the circadian system does not have to wait for rods and cones to report light. The ipRGCs can sense light themselves, and they also receive input from the outer retina, so they act as a light-detection hub that feeds the brain's clock [3]. In humans, ipRGC function has been studied indirectly through melatonin suppression and the pupillary light reflex, and more recently through direct descriptions of the cells' morphology and light responses [4].
The pathway from eye to clock
The ipRGCs send their signals along a specific bundle of nerve fibers called the retinohypothalamic tract, which projects to the brain region that houses the circadian clock [3]. This tract is the anatomical route by which light reaches the clock. Because it is separate from the main visual pathway, it can keep working even when the image-forming system is badly degraded.
The same non-image-forming system also drives light-induced suppression of melatonin, the hormone that signals biological night [3]. That is why a blind person with no conscious light perception can still have their melatonin production dialed down by bright light, and why the timing of melatonin can stay synchronized to the day-night cycle.
What happens when cones are missing
Congenital achromatopsia is a condition in which the cone system is non-functional. People with it are extremely light-sensitive and often avoid daytime light, which means their eyes receive much less light overall [2]. In a case series of seven genetically confirmed patients, researchers found regular rest-activity patterns in all of them and normal phase angles of entrainment—the timing relationship between the melatonin rhythm and sleep—in those with a measurable dim-light melatonin onset [2]. The group as a whole showed a tendency toward a late chronotype and some sleep problems, but the core finding was that behavioral and hormonal entrainment remained regular despite the lack of functioning cones [2].
This is consistent with the ipRGC pathway doing the essential work of entrainment. The patients' severe light avoidance attenuated their light exposure, yet their rhythms stayed organized, suggesting that the remaining light-sensing cells were sufficient [2]. The study is a small case series, so it describes what was observed in these seven people rather than proving the mechanism in every person with cone dysfunction.
Why blue light matters, and where uncertainty remains
Melanopsin is most sensitive to short wavelengths, roughly blue light in the 460–480 nm range [5]. That spectral sensitivity is the basis for practical interventions: in a small study of healthy adults, wearing glasses that block wavelengths below 500 nm prevented bright light from suppressing melatonin at night [5]. The glasses reduced light intensity by about half, and melatonin levels under the glasses were not significantly different from dim-light conditions [5]. This supports the idea that the ipRGC-melanopsin system is a major route for light's effect on melatonin, though the study was small and short-term.
Not every light response is purely melanopsin-driven, however. In studies of the pupillary light reflex under continuous light, the expected short-wavelength signature of ipRGCs was not always prominent, and cone photoreceptors appear to contribute under some conditions [1]. That does not undermine the role of ipRGCs in entrainment and melatonin suppression; it means the non-image-forming system integrates signals from multiple receptors depending on the lighting context [1][3]. For a blind person or a cone-deficient mouse, the ipRGCs provide the essential light input, but the fine details of how other receptors modulate that input are still being worked out.
Sources used in this answer
Ocular light effects on human autonomous function: the role of intrinsically photosensitive retinal ganglion cell sensitivity and time of day
In healthy humans, light effects on cardiac effort and the pupillary light reflex did not consistently show the expected short-wavelength ipRGC signature under continuous light, suggesting cones also contribute to nonvisual light responses depending on lighting conditions.
Sleep and circadian phenotype in people without cone-mediated vision: a case series of five<i>CNGB3</i>and two<i>CNGA3</i>patients
In seven patients with congenital achromatopsia and non-functional cones, rest-activity patterns were regular and melatonin phase angles of entrainment were normal in those with measurable dim-light melatonin onset, despite severely attenuated light exposure.
Comparative Neurology of Circadian Photoreception: The Retinohypothalamic Tract (RHT) in Sighted and Naturally Blind Mammals
The retinohypothalamic tract, driven by melanopsin-containing intrinsically photosensitive retinal ganglion cells, mediates circadian entrainment, melatonin suppression, the pupillary reflex, and sleep/wake effects in both sighted and naturally blind mammals.
Intrinsically Photosensitive Retinal Ganglion Cells of the Human Retina
Human intrinsically photosensitive retinal ganglion cells expressing melanopsin play critical roles in non-image-forming vision, including circadian entrainment and melatonin suppression, and their responses have been characterized indirectly through melatonin suppression and the pupillary light reflex.
Blue Light-Blocking Glasses Attenuate Light-Induced Melatonin Suppression in Healthy Japanese Adults
In seven healthy adults, blue light-blocking glasses that filter wavelengths below 500 nm attenuated bright-light-induced melatonin suppression, with melatonin levels under the glasses not significantly different from dim-light conditions.