SignificanceCircadian clocks drive daily physiological rhythms that adapt us to day and night. Their disruption by societal pressures (shift-work) or disease (neurodegeneration) carries severe economic and health consequences. The suprachiasmatic nucleus (SCN) is the principal brain clock, coordinating rhythms across the body. It consists of interconnected neurons that use the inhibitory neurotransmitter GABA, alongside astrocyte supporting cells. Using microscopic imaging, pharmacology, and molecular genetic approaches, we show that astrocytes sustain SCN timekeeping by controlling a daily rhythm of GABA levels. Their reduction during the day, by enhanced clearance, provides a circadian window for neuronal activity. This resolves a long-standing paradox of how an exclusively inhibitory network sustains neuronal activity and highlights the critical importance of astrocyte-to-neuron signaling in the clock.