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 (Pos…