High-Definition Transcranial Direct Current Stimulation During Sleep.

High-definition transcranial direct current stimulation (HD-tDCS) enables high spatial precision modulation of neural activity during sleep. This protocol presents a comprehensive methodology for applying HD-tDCS using a 4 x 1 ring electrode configuration while simultaneously recording high-definition electroencephalography (HD-EEG) to monitor sleep architecture and stimulation effects. The technique enables systematic tuning of multiple parameters, where each parameter selection produces distinct neurophysiological outcomes. This provides opportunities to target specific brain regions and oscillatory patterns specifically tailored to the research question. Successful implementation of this protocol requires attention to electrode placement, impedance management, and real-time sleep staging capability. The versatility of this methodology and its capacity to address causal relationships in sleep physiology invite a broad range of applications from basic neuroscience to clinical interventions based on sleep optimization. The standardized yet flexible framework described here establishes HD-tDCS as a robust platform for systematic investigations of sleep neurobiology as well as its clinical ramifications.