A single-site, randomized study to assess the effectiveness of different types of closed-loop auditory stimulation on slow-wave enhancement using SmartSleep in an astronaut-like sample of adults between the ages of 30 and 55. Secondary analyses will be performed to determine whether slow-wave enhancement influences neurobehavioral performance in this population. Participants will be on study for 8 weeks.

The investigators seek to determine the extent to which various closed-loop auditory stimulation protocols (continuous fixed interval, block, in-phase adjustable) differentially affect slow-wave sleep. Participants will run the study for eight weeks, which includes wearing the SmartSleep device nightly, and completing one 3-min and one 20-min cognitive assessment battery each day, as well as computerized post-sleep questionnaires each morning, sleepiness scales three times a day, and a weekly evaluation of device comfort. This study will employ various types of auditory stimulation in order to determine 1) which stimulation pattern is maximally effective at enhancing slow wave activity (SWA) and 2) to assess cognitive performance associated with each stimulation algorithm. The study requires a longer duration so that sufficient data on each distinct type of stimulation may be collected.

Participants will be instructed to wear the SmartSleep device at home for three two-week periods, and will complete 10 nights per period of continuous fixed interval, block, or in-phase adjustable stimulation (in randomized order). Between each of these periods, participants will complete one week of sham condition (this condition records sleep EEG without any stimulation and is used to define each participant's typical sleep). During all eight weeks, participants will complete a 20-minute cognitive assessment in the evening, a five-minute cognitive assessment in the morning, and respond to the Karolinska Sleepiness Scale three times per day. The cognitive assessments will be accompanied by short questionnaires. Participants will receive text message reminders from study staff when it is time to perform these assessments.

Primary objective: Determine which stimulation type (e.g., Block stimulation, Fixed ITI, in-phase) is most effective at enhancing slow-waves slow-wave sleep relative to baseline sleep in a group of astronaut-like subjects in an ecologically valid setting.

Main secondary objective: Determine the extent to which various closed-loop auditory stimulation protocols (continuous fixed interval, block, inphase adjustable) differentially affect various aspects of neurobehavioral function using a set validated cognitive tasks as part of the Cognition battery.

Other secondary objectives: Explore the relationships between improvements in SWA and measures of subjective sleepiness as assessed by the Karolinska Sleepiness Scale

Inclusion Criteria:

• Able to provide written informed consent
• Able to understand and speak English
• Left or right-handed participants may be included
• Participants have access to a cell phone that can receive text message reminders from study staff
• Participants have self-reported sleep latency (time taken to fall asleep) > 30 minutes no more than once per week
• Participants have self-reported wake time after sleep onset ≤ 30 minutes
• Participants with Body Mass Index (BMI) < 35 kg/m2 AND
• Participants self-report using an alarm to wake up ≥ 5 days a week
• Participants self-report sleeping one or more hours longer on weekends (or when given the opportunity) than they do on a typical work night

Exclusion Criteria:

• Participants who self-report taking medications that could affect sleep patterns
• Participants who have had travel in the last 2 weeks or who intend to travel during the experimental weeks with time zone shifts >3h
• Participants unlikely to comply with the protocol (e.g., uncooperative attitude)
• Individuals who self-report a current severe or chronic medical condition or sleep disorder that may affect sleep patterns
• Individuals who self-report a history of recurrent seizures or epilepsy or family history of hereditary epilepsy or have a history of medical conditions that could increase the chance of seizures (e.g. stroke, aneurysm, brain surgery, structural brain lesion)
• Participants who report not being consistent in their daily use of alcohol, caffeine, or nicotine
• Participants who are pregnant
• Participants who are night shift workers
• Individuals who self-report severe contact dermatitis or allergy to silicone, nickel or silver
• Individuals who self-report moderate hearing loss
• Self-reported history of excessive alcohol intake- selfreport > 21 drinks / wk or binge alcohol consumption ( >5 drinks per day)
• Excessive caffeine consumption (> 7 cups) combining all caffeinated drinks regularly absorbed during workdays. Caffeine intake must be regular and maintained throughout study and on testing days
• Participants who do not have internet access at home
• Participants who express unwillingness to forego napping on weekdays during the study
• Participants who smoke
• Participants with high risk of Obstructive Sleep Apnea (OSA) based on STOP-BANG Questionnaire ("yes" on at least 4 of 8 questions)
• High Risk of Restless Legs Syndrome (RLS) based on the short form Cambridge-Hopkins Screening questionnaire
• High Risk of Insomnia based on Insomnia Severity Index (score of 22 or higher)

Participants will be blind to the device condition