The study by Massimini et al. investigates the spatiotemporal dynamics of slow oscillations during non-rapid eye movement (NREM) sleep in humans using high-density electroencephalogram (EEG) recordings. They found that each cycle of the slow oscillation, which occurs at a frequency of <1 Hz, is a traveling wave that originates at a specific site and propagates over the scalp at a speed of 1.2–7.0 m/sec. The waves originate more frequently in prefrontal-orbitofrontal regions and propagate in an anteroposterior direction. The rate of occurrence of these oscillations increases progressively as sleep deepens, reaching almost once per second in the deepest stages of NREM sleep. The pattern of origin and propagation is reproducible across nights and subjects, providing insights into cortical excitability and connectivity. The orderly propagation of correlated activity along connected pathways may play a role in spike timing-dependent synaptic plasticity during sleep.The study by Massimini et al. investigates the spatiotemporal dynamics of slow oscillations during non-rapid eye movement (NREM) sleep in humans using high-density electroencephalogram (EEG) recordings. They found that each cycle of the slow oscillation, which occurs at a frequency of <1 Hz, is a traveling wave that originates at a specific site and propagates over the scalp at a speed of 1.2–7.0 m/sec. The waves originate more frequently in prefrontal-orbitofrontal regions and propagate in an anteroposterior direction. The rate of occurrence of these oscillations increases progressively as sleep deepens, reaching almost once per second in the deepest stages of NREM sleep. The pattern of origin and propagation is reproducible across nights and subjects, providing insights into cortical excitability and connectivity. The orderly propagation of correlated activity along connected pathways may play a role in spike timing-dependent synaptic plasticity during sleep.