Abstract Introduction Sleep disturbances and cognitive deficits are hallmark, debilitating features in schizophrenia. High-definition transcranial direct current stimulation (HD-tDCS) is a promising non-invasive approach to modulate restorative sleep and sleep-dependent memory consolidation, potentially addressing cognitive impairments that are resistant to conventional pharmacological interventions. Methods Ten healthy participants underwent within-subject, counterbalanced nap sessions with either bifrontal HD-tDCS or sham stimulation delivered during Stage 2 (N2) sleep. High-density electroencephalographic recordings enabled characterization of spectral power and temporal dynamics in key sleep oscillations: SO (0.5-1 Hz), slow spindles (9-12 Hz), and fast spindles (12-15 Hz). Pre- and post-nap cognitive assessments evaluated procedural memory (motor sequence task) and declarative memory (word-pair associates). Results In contrast to sham controls, HD-tDCS significantly increased frontal SO power (p = 0.02, r = -0.818) in the interval immediately following stimulation without altering spindle density, amplitude, duration, or power. However, the refractory period between spindle events in the period following completion of the stimulation protocol was significantly reduced (p = 0.041, r = 0.588) in the active HD-tDCS condition, unlike the sham controls. SO-spindle coupling remained unchanged. Behaviorally, active HD-tDCS enhanced procedural memory consolidation (p = 0.010, r = -0.891) but not declarative memory performance. Conclusion Targeted HD-tDCS during nap-time sleep amplifies SO and modulates spindle temporal dynamics with observed enhancements in procedural memory consolidation. Notably, core features of spindles remained intact: their density, amplitude, duration, or power as well as their coupling with SO. This suggests that active neuromodulation during sleep may prime key thalamocortical circuitry for more frequent spindle generation without altering intrinsic spindle characteristics or fundamental temporal coordination underlying memory consolidation. These preliminary results establish a mechanistically grounded foundation for developing high-precision, sleep-based interventions with potential to correct pathologies in sleep architecture and cognition in schizophrenia. Support (if any)
Song et al. (2026) studied this question.