Modeling circadian rhythm splitting under chronic light and the asymmetric transition kinetics between lighting conditions

Every living creature, from the smallest mouse to the human being, carries an internal clock that ticks with a rhythm of roughly twenty-four hours. This biological timekeeper, known as the circadian rhythm, dictates when we sleep, when we eat, and when our bodies are most alert. In mammals, this master clock is located in a tiny, paired structure deep within the brain called the suprachiasmatic nucleus, or SCN. Under normal conditions, with the sun rising and setting each day, the two halves of this structure work in perfect unison, sending a single, coordinated signal to the rest of the body to maintain a steady daily cycle. However, when animals are exposed to constant, unending light for weeks or months, something strange happens: their single daily period of activity splits into two distinct bouts, separated by about twelve hours. The animal begins to act as if it has two days and two nights within a single twenty-four-hour cycle. For decades, scientists have observed this phenomenon, known as "splitting," across many species, yet the precise mechanism that causes the brain to fracture its own timekeeping has remained a mystery.

A new study by researchers Alexander Goltsev and José Mendes offers a detailed explanation for this puzzling behavior, suggesting that the split is not a malfunction, but a sophisticated reorganization of how the brain's two halves communicate. The scientists built a computer model of the SCN to simulate how its neurons interact under different lighting conditions. They treated the brain structure as two lobes, left and right, each containing two distinct groups of cells: a ventral core and a dorsal shell. In the darkness, these groups talk to each other using chemical signals that encourage them to act together, keeping the entire system synchronized. The researchers propose that when an animal is bathed in constant light, the chemical nature of these signals flips. Instead of encouraging unity, the signals begin to push the groups apart, forcing the left and right sides of the brain to oscillate in opposite phases. One side becomes active while the other rests, and then they switch roles twelve hours later. Because the shell regions of the brain are responsible for sending timing signals to the rest of the body, this back-and-forth switching creates the two separate peaks of activity observed in split animals.

The key to this flip-flop lies in a microscopic switch inside the neurons that controls how they respond to a common neurotransmitter called GABA. Normally, GABA acts as a brake, calming neurons down. However, the effect of GABA depends on the balance of specific salt-transporting proteins on the surface of the cell. In the dark, these proteins are arranged so that GABA acts as a stimulant, helping the two halves of the brain synchronize. Under constant light, the expression of these proteins changes over time, altering the internal chemistry of the cell. This shift turns GABA from a stimulant into a suppressor. The researchers suggest that this change in chemical polarity is what causes the connections between the brain lobes to reverse, turning a cooperative relationship into a competitive one where the two sides fight for dominance, resulting in the split rhythm.

To test this idea, the team simulated the behavior of the SCN under various conditions. When they set the model to mimic constant darkness, the two lobes synchronized perfectly, producing a single, steady rhythm. When they introduced the light-induced chemical changes, the model spontaneously split, with the left and right lobes moving in opposite directions. The simulation showed that this split state produces two activity peaks exactly twelve hours apart, matching real-world observations. The model also reproduced what happens when one half of the brain is damaged. In experiments where one lobe of the SCN is destroyed, the split rhythm disappears, and the remaining healthy lobe takes over, returning the animal to a single, unified daily cycle. The computer model mirrored this result, showing that without the opposing lobe to compete with, the remaining side stabilizes into a normal, single-bout rhythm. This confirms that the split state is a result of the interaction between the two lobes, not a defect within a single lobe.

One of the most intriguing aspects of the split rhythm is how long it takes to appear and how quickly it disappears. When an animal is first moved to constant light, it can take anywhere from thirty to over a hundred days for the rhythm to split. Yet, if the animal is returned to darkness, the split vanishes in just a few days. If the animal is then returned to constant light, the split reappears much faster than the first time. The researchers incorporated the slow, biological process of changing the salt-transporting proteins into their model to explain this delay. They found that the time it takes for these proteins to build up or break down naturally accounts for the slow onset of splitting and the rapid recovery. The model suggests that the system has a kind of inertia; it takes a long time for the chemical environment to shift enough to flip the switch, but once the switch is flipped, the system can revert quickly when the light is removed. This kinetic asymmetry, where the path to splitting is slow but the path back is fast, is a direct consequence of how the cells manage their internal chemistry.

The study also explored the specific timing of the split. In the split state, the core and shell regions within each individual lobe are also slightly out of step with one another, but the most dramatic effect is the opposition between the left and right lobes. The model showed that the shell regions, which control the body's behavior, send signals that are exactly twelve hours apart. This means that while the left side of the brain is signaling "day," the right side is signaling "night," and vice versa. The combination of these two opposing signals creates the double-peaked activity pattern seen in the animal's behavior. The researchers noted that this split state is stable and self-sustaining as long as the constant light continues, but it is fragile; removing the light or damaging one lobe immediately restores the single, unified rhythm.

This work provides a concrete, biophysical explanation for a phenomenon that has been observed for over sixty years. By linking the macroscopic behavior of the animal to the microscopic changes in salt transport within individual neurons, the researchers have connected the dots between light, chemistry, and timekeeping. They suggest that the brain does not simply break under constant light but rather adapts by reconfiguring its internal communication network. The shift from a cooperative, excitatory mode to a competitive, inhibitory mode allows the two halves of the brain to maintain their own rhythms while preventing them from collapsing into a single, chaotic signal. While the model is a simulation and relies on assumptions about how certain proteins behave, it aligns closely with experimental data from rats and mice, offering a compelling framework for understanding how the brain's master clock can be so flexible. The findings highlight the plasticity of the circadian system, showing that our internal timekeeping is not a rigid machine but a dynamic network capable of profound reorganization in response to the environment.