IntroductionAging impairs the function of the central circadian clock in mammals, the suprachiasmatic nucleus (SCN), leading to a reduction in the output signal. The weaker timing signal from the SCN results in a decline in rhythm strength in many physiological functions, including sleep–wake patterns. Accumulating evidence suggests that the reduced amplitude of the SCN signal is caused by a decreased synchrony among the SCN neurons. The present study was aimed to investigate the hypothesis that the excitation/inhibition (E/I) balance plays a role in synchronization within the network.MethodsUsing calcium (Ca2+) imaging, the polarity of Ca2+transients in response to GABA stimulation in SCN slices of old mice (20–24 months) and young controls was studied.ResultsWe found that the amount of GABAergic excitation was increased, and that concordantly the E/I balance was higher in SCN slices of old mice when compared to young controls. Moreover, we showed an effect of aging on the baseline intracellular Ca2+concentration, with higher Ca2+levels in SCN neurons of old mice, indicating an alteration in Ca2+homeostasis in the aged SCN. We conclude that the change in GABAergic function, and possibly the Ca2+homeostasis, in SCN neurons may contribute to the altered synchrony within the aged SCN network.