The slow oscillation is generated through the rhythmic generation and failure of the persistent "UP: states in the cerebral cortex

Figure 11.  The "UP" state of the slow oscillation is generated through recurrent excitation that is controlled by local inhibition.  This activity allows for the entry of Ca2+ and Na+ into the neuron, which in turn results in the activation of K+ currents.  The build up of these K+ currents results in the failure of the slow oscillation, and the cessation of the "UP" state.  During the "DOWN" state, the K+ currents slowly dissipate, and this allows for the generation of another "UP" state.  Through these mechanisms, the cerebral cortex during sleep generates rhythmic, recurrent and spontaneous activity.