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

A two-lobe biophysical SCN model shows chronic light shifts GABA-driven excitation/inhibition so left and right shells oscillate in antiphase, yielding a 12-h rhythm.

• Why it matters: Chronic light splits the 24-h circadian activity bout into two 12-h components in vertebrates, but the mechanism linking lighting to SCN coupling remains unresolved.
• What they did: The authors model the SCN as interacting ventral core and dorsal shell Kuramoto-type oscillator populations across bilateral lobes, with time-varying chloride cotransporter expression that flips coupling signs.
• The result: Under constant darkness excitation dominates and populations synchronize in-phase, while under chronic light inhibition dominates, producing negative ipsilateral/contralateral couplings and antiphase shell oscillations that match experimental data.

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