Mapping Human Sleep-State Brain Function with Simultaneous EEG and Neuroimaging: A systematic review

Sleep is a dynamic process involving complex changes in brain activity across distinct stages, yet the neurobiology of these transitions remains poorly understood. Simultaneous electroencephalography (EEG) and neuroimaging provide a unique window into stage-specific brain activity by combining the temporal precision of EEG with the spatial resolution of functional Magnetic Resonance Imaging (fMRI), Positron Emission Tomography (PET), and the signal specificity of Near-Infrared Spectroscopy (NIRS). We conducted the first systematic review of simultaneous EEG-neuroimaging studies of human sleep. Systematic searches across five databases identified 205 eligible studies. Eligible studies recorded simultaneous EEG and fMRI, PET, or NIRS during sleep in humans, with EEG-based staging; sequential acquisition and awake-only studies were excluded. Findings were grouped by imaging modality, sleep state (NREM, REM, transitions), sleep manipulation type, and population (healthy vs. clinical). The majority used EEG-fMRI (n=141), with smaller numbers employing EEG-PET (n=34), EEG-NIRS (n=26), and multimodal combinations (n=4). Despite heterogeneity in protocols and analyses, convergent findings highlight robust thalamic, precuneus and cingulate involvement across non-REM (NREM) stages, with additional modality-specific insights. REM sleep showed increased activity in limbic and paralimbic networks, alongside reduced activity in executive control regions. Studies varied in design (49% overnight-only, 20% nap-only), populations (74% healthy adults only), and staging approaches (61% using 30-sec epochs), with methodological challenges including small sample sizes (median N=18) and heterogeneity in sleep manipulation protocols (28% used sleep restriction). This systematic review synthesizes three decades of simultaneous EEG-neuroimaging, mapping convergent and divergent findings across modalities to advance understanding of sleep-state brain function.