As people fall asleep, the brain's default mode network (DMN) and its anticorrelated network (ACN) break down. In 25 healthy participants, functional connectivity between key brain regions—especially the posterior cingulate cortex, parahippocampal gyrus, and medial prefrontal cortex—decreased with deeper non-REM sleep. The loss of synchronization between the posterior and anterior midline nodes of the DMN, and between the DMN and ACN, suggests that preserved corticocortical connectivity is necessary for maintaining internal and external awareness. The posterior cingulate/retrosplenial cortex appears particularly important for regulating consciousness.
Lucid dreaming—being aware that one is dreaming—is associated with reactivation of brain areas that are normally deactivated during REM sleep. In one experienced lucid dreamer who had two episodes of verified lucid REM sleep long enough for fMRI analysis, the bilateral precuneus, cuneus, parietal lobules, and prefrontal and occipito-temporal cortices showed strong activation compared with non-lucid REM sleep. This pattern may explain the return of reflective cognitive abilities that characterize lucid dreaming.