A genetically encoded ionic-stress sensor reveals protons as a sleep driver

Dynamic ionic changes are hallmarks of physiological and behavioral state transitions, including sleep in animals. Although biosensors for specific cellular ions are widely available, real-time monitoring of overall ionic strength in living organisms remains challenging. Here, we present a genetically encoded nuclear translocation ionic sensor (GENTIS) that enables direct visualization of ionic stress <i>in vivo</i>. Using GENTIS in <i>C. elegans</i>, we uncover rhythmic elevations in ionic strength during larval molting transitions that coincide with the lethargus sleep. Cytosolic proton ionic increase through inhibition of v-ATPase is sufficient to induce GENTIS nuclear translocation and evoke behavioral quiescence, characterized by reduced feeding and activation of sleep-active neurons. Apical membrane v-ATPases undergo disassembly during lethargus and under sleep-inducing stress conditions, leading to proton accumulation. Notably, this proton-mediated sleep is suppressed by proton buffering with ammonium. Together, these findings establish GENTIS as a powerful tool for tracking ionic strength dynamics in living organisms and support protons as a physiological driver of sleep.