Your eyes do not see the world the same way at midnight that they do at noon, and neither does your brain. A new preprint from researchers at the University of Manchester shows that the brain’s internal circadian clock actively rewires how the visual system communicates with itself, tuning the neural code for day and night. Published July 28 on bioRxiv, the study offers the clearest evidence yet that circadian rhythms are not passive background processes but active sculptors of moment-to-moment perception.
The finding comes from a surprising place: the dorsal lateral geniculate nucleus, or dLGN, a small relay station buried deep in the thalamus. Most textbook diagrams treat the dLGN as a simple switchboard, a passive relay that shuttles signals from the retina to the primary visual cortex without adding much of its own. The Manchester team’s data suggest that picture is wrong. Using wireless electrophysiology in awake, freely behaving mice, they recorded neural activity directly from the dLGN across the full circadian cycle and found something the textbooks never predicted: the nucleus’s spontaneous firing rate rises and falls on a precise daily schedule, peaking in the late subjective day regardless of whether the animal is in light or darkness.
That shift in baseline activity matters because spontaneous firing is the canvas on which visual signals are painted. When the canvas itself brightens or dims, the same visual input can produce a very different neural response. And that is exactly what the researchers observed. But the circadian effect is not a simple volume knob. The fundamental tuning properties of dLGN neurons (their sensitivity to orientation, contrast, and spatial frequency) stay remarkably stable around the clock. The neurons still prefer the same kinds of visual features whether it is day or night. What changes is not what they respond to, but how efficiently they encode it.
This is where the study’s second major finding comes into focus. The Manchester team applied tools from information theory to quantify exactly how many bits of visual information each action potential carried at different circadian phases. They discovered that the circadian system reconfigures the visual code in a context-dependent way. Under high-contrast conditions, such as a bright sunny landscape or a well-lit room, dLGN neurons adopted a more efficient coding strategy at night, packing more information per spike than they did during the day. Under low-contrast conditions, such as dusk or heavy shadow, the efficiency advantage disappeared.
The implication is elegant. The visual world humans and mice navigate looks very different at dawn than at noon. Shadows are deeper, edges are softer, contrast is lower. A sensory system that treats all lighting conditions the same way is wasting energy at best and misrepresenting the world at worst. What the dLGN appears to do instead is match its coding strategy to the statistics of the visual scene, and the circadian clock provides the rough schedule for that matching. The brain does not wait for the light level to change before adjusting its code. It anticipates the change, preparing the visual relay for different operating conditions before the sun even sets.
To confirm that the effect was truly circadian and not simply a response to light or dark exposure, the researchers ran a critical control experiment. They used optogenetics to silence retinogeniculate transmission, effectively cutting off input from the retina to the dLGN. Even with the retinal input blocked, the circadian modulation of spontaneous activity persisted. The clock driving these changes is internal to the thalamus, or at least independent of incoming visual drive. That finding pushes the locus of circadian control deeper into the brain than previously appreciated and raises the possibility that similar mechanisms exist in other sensory relays.
If confirmed, the implications reach beyond vision. The same kind of circadian reconfiguration could operate in the auditory thalamus, the somatosensory thalamus, and beyond. Every sensory system that passes through a thalamic relay may have a built-in daily rhythm that shifts how it encodes the world. That would mean the brain’s internal clock does not just govern sleep-wake cycles, hormone release, and metabolism. It also actively shapes perception itself, determining not just whether you are alert enough to see but how your visual system actually processes what is in front of you.
The study is a preprint, meaning it has not yet undergone peer review, and the findings come from mice, not humans. The mouse visual system differs from the human one in important ways: mice have far lower visual acuity, limited color vision, and a dLGN that is organized differently from the primate equivalent. Whether the same circadian reconfiguration occurs in the human thalamus remains an open question, though the fundamental biology of the circadian clock is highly conserved across mammals. The study is also correlative in part, and while the optogenetic experiment provides strong causal evidence for the circadian origin of the spontaneous activity changes, the molecular mechanism linking clock genes to neural firing in the dLGN has not yet been identified.
Still, this is the kind of paper that shifts how a field thinks about a structure it thought it understood. The dLGN was supposed to be a relay station. It turns out to have a clock of its own, ticking through the day and night, quietly rewriting the language in which vision speaks to the cortex.
Source: Pienaar A, Rodgers J, Storchi R, Allen AE. Circadian modulation of visual coding in the dorsal lateral geniculate nucleus. bioRxiv, July 28, 2026. DOI: 10.64898/2026.07.24.740496. CC-BY 4.0. Funded by the Wellcome Trust.