Transcranial Electrical Stimulation (TES) at Slow Oscillation (SO) Frequency During NREM Sleep

Inclusion Criteria: * Healthy men and non-pregnant, non-lactating women 18 to 39 years of age (inclusive) * Must demonstrate adequate comprehension of the protocol, by achieving a score of at least 80% correct on a short multiple-choice quiz. Individuals who fail to achieve a passing score on the initial quiz will be given one opportunity to retest after a review of protocol information. Individuals who fail the comprehension assessment for the second time will be disqualified. Exclusion Criteria: * Self-reported habitual nightly sleep amounts outside the target range of 6 - 9 hours (i.e., less than 6 hours per night or more than 9 hours per night, on average) (Post-consent Checklist) * Self-reported nighttime lights-out times earlier than 2100 hours on average during weeknights (Sunday through Thursday) or later than 2300. (Post-consent Checklist) * Self-reported morning wake-up times later than 0800 on average during weekdays (Monday through Friday) (Post-consent Checklist) * Self-reported habitual napping (\> 1 time a week in conjunction with normal sleep habits) (Post-consent Checklist) * A rating of 6 or below on question 2 or 3 of the Nonrestorative Sleep Scale, indicating the subject experiences relatively non-refreshing sleep * An average time to sleep onset of greater than 20 minutes as indicated on the Post-consent Checklist * Self-reported caffeine use in excess of 400 mg (e.g., approximately 8 caffeinated sodas or 4 12-oz cups of coffee) per day on average (Post-consent Checklist; document provides exclusionary limits for various caffeinated products). * Score of lower than 31 or higher than 69 on the Morningness-Eveningness Questionnaire (MEQ form) * Score of 14 or above on the Beck Inventory Form (BDI form) * Score of 41 or above on the Self-Evaluation Questionnaire * History of cardiovascular disease (to include but not limited to arrhythmias, valvular heart disease, congestive heart failure, history of sudden cardiac death or myocardial infarction) (Medical History and Examination Form) * History of neurologic disorder (to include, but not limited to epilepsy or another seizure disorder, amnesia for any reason, hydrocephalus, MS, narcolepsy or other sleep disorders) (Medical History and Examination form; sleep items on Post-Consent Checklist) * Underlying pulmonary disease requiring daily inhaler use (Medical History and Examination form) * Kidney disease or kidney abnormalities (Medical History and Examination form, laboratory results) * Liver disease or liver abnormalities (Medical History and Examination form, laboratory results) * Self-reported history of psychiatric disorder requiring hospitalization or psychiatric product within the last 2 years or for more than 3 months at one time. (Medical History and Examination form) * Self-reported or suspected regular nicotine use or addiction, defined as more than 1 cigarette or equivalent per week, within the last 1 year (Medical History and Examination form) * Self-reported or suspected heavy alcohol use; minimum limit to define heavy alcohol use is 14 drinks per week or as determined by the examining appropriately licensed study investigator (Medical History and Examination form) * Self-reported or suspected use of products or drugs that cannot be safely discontinued during in-laboratory phases, to be determined on a case-by-case basis by the examining appropriately licensed study investigator (Medical History and Examination form) * Self-reported or suspected current use of other illicit drugs, to include but not limited to benzodiazepines, amphetamines, cocaine, marijuana (Medical History and Examination form) * Positive urine pregnancy result * Resting blood pressure above 140/90 or resting pulse \> 110 (Medical History and Examination form). Note that if a repeat measurement is within range then the volunteer will not be excluded. * BMI ≥ 30 (Obese Class I or greater) (Medical History and Examination form) * Clinically significant values (as determined by the appropriately licensed study investigator reviewing the study) for any hematology or chemistry parameter. The appropriately licensed study investigator reviewing the laboratory values may opt to repeat any clinically significant tests and include volunteers whose repeat test values are not clinically significant. * Positive urine nicotine/cotinine result during screening visit (NicCheck™ I test strip results) * Positive urine drug result during screening visit * Any use of sleep aids during the 1 year prior to screening * Inability to read and sign consent * Lack of access to a quiet, dark environment conducive to sleep from 2100 until 0700 during a seven night period at the beginning of the study * Participation in any ongoing clinical trials. * The preceding exclusionary criteria are known to alter sleep (e.g., epilepsy; some neurological disorders), substantially increase inter-subject variability, and/or the disease or condition puts the subject outside the range of what is considered healthy. The PI also maintains the prerogative to disqualify a volunteer if it is deemed that the volunteer's participation would be unsafe for the volunteer or staff or would be disruptive to study conduct or their inclusion could compromise data integrity. * Volunteers meeting the Beck cut-off (a score of at least 14) and who carry health insurance will be instructed to call their health insurance Mental Health / Substance Abuse referral number. * Volunteers meeting the first cut-off (score of at least 14 who are not insured will be provided with a community mental health referral contact specific to their county of residence. * The review of the medical history with the volunteer and the physical examination itself will be performed by an appropriately licensed study investigator. Results of screening urine and blood tests will be reviewed by an appropriately licensed study investigator. The Post-consent Checklist will be administered by a trained research technician and r