RecruitingDevice studyOvernight Thalamic TES-TI to Modulate Sleep Spindles in Individuals With Schizophrenia Spectrum Disorders and Matched Healthy ControlsThis study to find out whether a type of non-invasive electrical brain stimulation called transcranial electrical stimulation with temporal interference (TES-TI) can temporarily change brain activity during sleep-especially sleep spindles (brain rhythms in the \~8-16 Hz range). The investigators are focusing on the thalamus, a deep brain region that helps coordinate brain activity during non-REM sleep. Sleep spindles are often reduced in schizophrenia, so this study is to see whether TES-TI can change spindle activity in individuals with schizophrenia spectrum disorders (SSD) and in healthy adults. To study this, a structural MRI scan will be used to customize where the stimulation electrodes are placed, and then TES-TI will be applied during one of two overnight sleep lab visits while brain activity is recorded with high-density EEG and standard sleep sensors. The other overnight is a baseline/control night during which only sham stimulation is delivered. The goal is to determine whether TES-TI during sleep can increase spindle-frequency activity in this population.