Synaptic mechanisms that contribute to memory consolidation in the human brain remain largely unexplored. The formation of lasting memories critically relies on sleep1-3. During slow wave sleep, neocortical neurons exhibit characteristic membrane potential oscillations known as UP and DOWN states4-7. In this study, we show that sleep-like UP and DOWN states modulate axonal action potentials and temporarily enhance synaptic transmission between human layer 2 & 3 pyramidal neurons. In convergent microcircuit motifs, synaptic enhancement by synchronous UP and DOWN states leads to efficient recruitment of postsynaptic action potentials, which in turn results in long-term stabilization of synaptic strength. In contrast, synapses undergo lasting depression if presynaptic neurons fail to recruit postsynaptic action potentials. Taken together, our study offers a mechanistic framework in humans that can explain how precise coupling of neural activity to slow waves gates synaptic plasticity, which is the substrate of memory consolidation8-12. This framework could inspire improved brain stimulation strategies targeting memory performance13-18.