Inclusion Criteria:
* Cognitively normal (Telephone Interview for Cognitive Status (TiCS) ≥29))
* Moderate - severe OSA
* Currently on therapy for OSA and has received treatment for OSA for at least 3 months.
* Sustained compliance with continuous positive airway pressure (CPAP), which means using the machine at least 4 hours per night for five nights a week.
* Epworth Sleepiness Score (ESS) greater than or equal to 8 or excess daytime sleepiness as defined by the Functional Outcome Sleep Questionnaire or clinical complaint.
Exclusion Criteria:
* Documented diagnosis of chronic insomnia, or sleep onset insomnia based on Insomnia Severity Index (ISI)
* Documented diagnosis of circadian rhythm disorder
* Any current use of supplemental oxygen
* Other sleep-related breathing disorders (central sleep apnea, etc.) based on American Academy of Sleep Medicine (AASM) criteria
* Current shift work involving night shift (regular work between 12am and 6am or night shift) within the past 6 months
* Anticipated scheduled bariatric surgery within the next 3 months
* Chronic regular (\> 2 nights per week) use of any sedative, stimulant, neuroleptic drugs, or other medications limiting validity of cognitive tests. This includes regular use of alcohol or marijuana for sleep. Melatonin is ok.
* Diagnosis of uncontrolled psychiatric disease in the last six months, and/or history of schizophrenia or bipolar disorder. Controlled conditions will include major depressive disorder, panic disorder, schizoaffective disorder, generalized anxiety disorder, Obsessive-compulsive disorder (OCD), substance use disorders, and alcohol abuse/dependence. Personality disorders and neurodevelopmental disorders (e.g. autism, ADHD) are allowed if cognition is within normal limits.
* Taking methylphenidate for ADHD.
* Presence of other critical comorbid conditions that would lead to inability to complete the study protocol (including follow-up for 2 years), or that would affect cognition (e.g. clinically relevant endocrine or hematological conditions).
* Chronic regular (\> 2 nights per week) use of stimulant if unable to complete a washout prior to MRI.
* Does not have a regular sleeping environment (i.e., sleeps in a different setting \> 2 nights per week).
* Currently pregnant or planning to become pregnant.
* Prior diagnosis of a Central nervous system (CNS) disease, such as multiple sclerosis, stroke, Parkinson's disease, Alzheimer's disease, epilepsy, a loss of consciousness \> 24 hours, or traumatic brain injury as identified by the Cumulative Illness Rating Scale for Geriatrics (CIRS). Participants who are diagnosed with Mild cognitive impairment (MCI) or Alzheimer's disease based on neuropsychological testing will be excluded. Delirium in the last 12 months.
* Near-miss or prior automobile accident "due to sleepiness" within the past 12 months.
* Employed as a commercial driver during the study (for example, bus drivers, train engineers, airplane pilots) or construction worker.
* Past intolerance or hypersensitivity to tPBM.
* Significant skin conditions (i.e., hemangioma, scleroderma, psoriasis, rash, open wound or tattoo) on the subject's scalp that are found in the area of the procedure sites.
* Any use of light-activated drugs (photodynamic therapy) within 14 days prior to study enrollment.
* Any type of implants in the head, whose functioning might be affected by tPBM (e.g., stent, clipped aneurysm, embolized arteriovenous malformation (AVM), implantable shunt - Hakim valve).
* Failure to meet standard MRI safety requirements (e.g., claustrophobia, non-removable piercings, implanted medical devices, other non-removable metals) as determined by the MRI Safety Checklist.
