Non-invasive Brain Stimulation for Sleep Study

Inclusion Criteria: * Medically healthy (based on self-report and study team review) * U.S. citizen or holding permanent resident status * English-speaking (able to provide consent and complete questionnaires) Exclusion Criteria: * Any current or past history of neurological disorders or acquired neurological disease (e.g. stroke, traumatic brain injury), including intracranial lesions (including clinically significant findings identified in first MRI) * History of inpatient psychiatric hospitalization * History of head trauma resulting in prolonged loss of consciousness; or a history of greater than 3 grade I concussions * Current history of poorly controlled headaches including intractable or poorly controlled migraines * Any systemic illness or unstable medical condition that may cause a medical emergency in case of a provoked seizure (cardiac malformation, cardiac dysrhythmia, asthma, etc.) * History of seizures, diagnosis of epilepsy, history of abnormal (epileptiform) EEG, or family history of treatment resistant epilepsy except for a single seizure of benign etiology (e.g. febrile seizures) in the judgment of a board-certified neurologist * Possible pregnancy or plan to become pregnant in the next 6 months (self reported) * Any metal in the head * Any medical devices or implants (i.e. cardiac pacemaker, medication infusion pump, cochlear implant, vagal nerve stimulator) * Dental implants * Permanent retainers * Any hair braid, dreadlocks, hair pieces, or extensions which cannot be taken out before the study sessions * Any head coverings or headdress that participant feels uncomfortable removing for the purposes of study sessions * Any medication that may alter seizure threshold taken during the study i.e., ADHD stimulants (Adderall, amphetamine); Tricyclic/atypical antidepressants (amitriptyline, doxepine, imipramine, maprotiline, nortriptyline, bupropion); SSRIs (Escitalopram, Fluoxetine, Sertraline); Antipsychotics (chlorpromazine, clozapine), Bronchodilators (theophylline, aminophylline); Antibiotics (fluoroquinolones, imipenem, penicillin, cephalosporins, metronidazole, isoniazid); Antivirals (valacyclovir, ritonavir); OTC antihistamines (diphenhydramine, Benadryl) * Claustrophobia (a fear of small or closed places) * Back problems that would prevent lying flat for up to two hours * Regular night-shift work (second or third shift) * Sleep apnea or other sleep disorder (self-reported)