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 emotional health and stress resilience.

Participants will attend up to 3 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 is a single-blind, two-phase study. In the first phase, each participant will be assigned a nap length, a trigger region, a stimulation type (difference frequency, multipolar transcranial electrical stimulation with temporal interference (TES-TI), or phase modulation), and a stimulation start time relative to the appearance of spindles and sawtooth waves in their EEG. These parameters will be varied across and within participants who may undergo multiple naps in order to identify optimal parameters for enhancing REM sleep.

Phase II is a single-blind study comprised of 40 participants. Each participant will undergo two 90-minute naps with possible sham or non-sham stimulation conditions. The naps will be on 2 separate visits to the lab with at least 1 week between visits. Conditions on each visit are randomized and counterbalanced. During each visit, participants will complete both the REST-Q and SSS questionnaires. Full HDEEG will be applied as well as 4-16 stimulation electrodes. Participants will then undergo an emotion regulation task (face task) wherein they are presented with positive, negative, or neutral emotional stimuli. This is followed by a resting EEG recording, then a nap up to 90 minutes in duration. Either sham or non-sham stimuli will be applied at times during the nap period. Upon waking, the same questionnaires and emotion regulation tasks will be completed.

Inclusion Criteria:

• Adults aged 18-50 years
• Medically healthy
• English-speaking (able to provide consent and complete questionnaires)
• Citizen or holding permanent resident status
• Regular napper (1 or more naps per week)

Exclusion Criteria:

• Any current or past history of neurological disorders or acquired neurological disease (e.g. stroke, traumatic brain injury), including intracranial lesions
• History or head trauma resulting in prolonged loss of consciousness; or a history of >3 grade 1 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, histoy of abnormal (epileptiform) EEG, or family history of treatment resistant epilepsy with the exception of 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 body
• Any medical devices or implants (i.e. cardiac pacemaker, medication infusion pump, cochlear implant, vagal nerve stimulator) unless otherwise approved by the responsible MD
• Dental implants containing metal
• Any medication that may alter seizure threshold: ADHD stimulants (Adderall, amphetamine); tricyclic/stypical antidepressants (amitriptyline, doxepine, imipramine, maprotiline, nortriptyline, buproprion); 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)