During sleep, functional connections between brain networks shift in a directional manner. In non-REM (NREM) sleep, positive wake-like correlations often become negative and strengthen in the opposite direction, while in REM sleep they trend back toward positive correlations. This pattern supports the idea that NREM sleep involves altered, not merely reduced, functional connectivity. Many of these connections involve higher-order networks linked to cognition and consciousness, such as the default mode network, suggesting possible accompanying changes in cognitive and conscious states.
Co-occurring cannabis and tobacco use is associated with altered connectivity in the brain's default-mode network (DMN). Using resting-state functional MRI, researchers compared DMN connectivity in four groups: cannabis-dependent nonsmokers, cannabis-dependent tobacco smokers, tobacco-only smokers, and healthy controls. All substance-using groups showed lower DMN connectivity than controls, with distinct differences in connections involving the cerebellum, medial prefrontal cortex, parahippocampus, and anterior insula. Among cannabis-dependent individuals, stronger connectivity between the posterior cingulate cortex and right anterior insula correlated with longer duration of cannabis use. These findings suggest that both cannabis and tobacco exposure independently and jointly affect DMN function.