This study will involve three to five study visits over a 2- to 3-month period. During the first 3 days, participants will record their sleep habits in a diary and will wear a device that measures breathing, oxygen levels, and sleep position. All participants will then take part in a 1-night inpatient stay at the sleep center lab during which brain, eye, muscle, heart, and breathing activity will be monitored. Participants with a confirmed diagnosis of OSA will stay at the sleep lab one additional night for observation while using a CPAP machine. The following day, tests to measure alertness, ease of falling asleep, and sleepiness levels will be administered. Participants will then use the CPAP machine at home for 4-6 weeks; each machine will be set at an appropriate level for the participant, and will record breathing patterns and pressure. Participants will receive weekly phone calls for monitoring purposes. At the end of Week 4, participants will return to the sleep lab for one additional night of CPAP monitoring, followed by alertness and sleep testing. Those with severe OSA will use the CPAP machine at a newly adjusted level for 5 additional days and return for repeat testing. All participants who have been compliant with CPAP will return to the lab for overnight sleep testing without the CPAP machine and for alertness testing.