Annual changes in the environment threaten survival, and numerous biological processes in mammals adjust to this challenge via seasonal encoding by the suprachiasmatic nucleus (SCN). To tune behavior according to day length, SCN neurons display unified rhythms with synchronous phasing when days are short, but will divide into two sub-clusters when days are long. The transition between SCN states is critical for maintaining behavioral responses to seasonal change, but the mechanisms regulating this form of neuroplasticity remain unclear. Here we identify that a switch in chloride transport and GABA A signaling is critical for maintaining state plasticity in the SCN network. Further, we reveal that blocking excitatory GABA A signaling locks the SCN into its long day state. Collectively, these data demonstrate that plasticity in GABA A signaling dictates how clock neurons interact to maintain environmental encoding. Further, this work highlights factors that may influence susceptibility to seasonal disorders in humans., Competing Interests: KR, HP, SH, CK, DM, NR, JE No competing interests declared, (© 2019, Rohr et al.)