The goal of this clinical trial is to find out whether stimulating the brain with electrical current during naps can increase certain kinds of brain activity that happen during sleep and lead to improvements in mental fatigue.

Participants will attend 2 study visits, each of which may last up to 4-5 hours. During these visits, participants will wear a high density electroencephalography (hdEEG) cap and take a nap.

This project aims to use transcranial electrical stimulation with temporal interference (TES-TI) to enhance the production of sleep slow waves, boosting the restorative power of sleep and mitigating cognitive impairment (mental fatigue) and its underlying cause (brain fatigue).

Inclusion Criteria:

• Adults aged 18-75 of any gender identity
• Self-reported non-restorative sleep, as measured by a REST-Q score
• English-speaking (able to provide consent and complete questionnaires)
• Capable of taking daytime naps
• US Citizen

Exclusion Criteria:

• Any current or history of neurological disorders or acquired neurological disease (e.g. stroke, traumatic brain injury), including intracranial lesions

• History of inpatient psychiatric hospitalization

• History of head trauma resulting in prolonged loss of consciousness; or a history of >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

• 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); 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)