Inclusion Criteria:
To be eligible to participate in this study, an individual must meet all of the following criteria:
* Male and female subjects with normal cognition and 55-75 years.
* Within normal limits on neurological and psychiatric examinations. All subjects enrolled will have a CDR=0.
* All subjects will have had a minimum of 12 years of education. Among minority subjects \>80% of the elderly individuals coming to the NYU-ADC meet this criterion. The education restriction reduces performance variance on cognitive test measures and improves the sensitivity for detecting pathology and disease progression using the robust norms available at NYU. Given most subjects will meet this criterion we do not consider this a major selection bias or generalization limitation for this study.
* An informed family member or life-partner (preferably bed-partner) will be interviewed to confirm the reliability of the subject interview.
Exclusion Criteria:
An individual who meets any of the following criteria will be excluded from participation in this study:
* History of brain tumor, MRI evidence of brain damage or brain disease including significant trauma, hydrocephalus, seizures, mental retardation or other serious neurological disorder (e.g. Parkinson's disease or other movement disorders).
* Significant history of alcoholism or drug abuse.
* History of psychiatric illness (e.g., schizophrenia, bipolar, PTSD, or life-long history of major depression).
* Geriatric Depression Scale (short form)\>6.
* Insulin dependent diabetes.
* Evidence of clinically relevant cardiac, pulmonary, endocrine or hematological conditions.
* Physical impairment of such severity as to adversely affect the validity of psychological testing.
* Any prosthetic devices (e.g., pacemaker or surgical clips) that constitutes a hazard for MRI imaging.
* Medications affecting cognition: Narcotic analgesics, chronic use of medications with anticholinergic activity, anti-Parkinsonian medications (carbidopa/levodopa, amantadine, bromocriptine, selegiline). Others: amphetamines, amphetamine-like compounds, appetite suppressants, phenothiazines, reserpine, buspirone, clonidine, disulfiram, guanethidine, MAO inhibitors, theophylline, tricyclic antidepressants, salicylates, cholinesterase inhibitors and memantine.
* History of a first-degree family member with early onset (age \<60 years) dementia.
* Irregular sleep-wake rhythms (based on the actigraphy recordings) or severe OSA (AHI4%≥30).
* Chronic use of antidepressants and melatonin are allowed.
* Excessive daytimes sleepiness (Epworth Sleepiness Scale \>10) or history of CVE (arrhythmias, heart valve disease, cardiomyopathy, carotid or coronary artery disease and chest pain) will not be allowed in the OSA groups.
