Obstructive Sleep Apnea (OSA) is a sleep disorder characterized by impaired patency of the upper airway during sleep, resulting in numerous brief arousals from sleep and causing significant disability, lost quality of life and premature mortality1. OSA affects approximately 4-6% of the population nationwide 2, with higher rates among military Veterans 3, and an increase in prevalence of nearly twofold between 2005 and 2014 among VA patients4. OSA has direct effects on quality of life, health-care resources consumption and costs 5. Military Veterans who have been diagnosed with OSA have greater risk of developing depressive and anxiety disorders 6 possibly mediated by daytime consequences of poor sleep, such as fatigue and excessive daytime sleepiness7. While effective at treating respiratory disturbances and providing symptoms resolutions, treatment with Positive Airway Pressure (PAP) is not universally successful and a non-negligible proportion of patients who are correctly using PAP do not experience symptomatic relief from sleepiness and depression, suggesting that mechanisms beyond the respiratory disturbances may be involved in the pathogenesis of symptoms leading to disability. A growing body of literature in animal and human models suggests that the sleep and respiratory disturbances commonly seen in OSA, namely sleep fragmentation, partial sleep deprivation, intermittent hypoxia, can promote shifts in circadian rhythms ultimately leading to misalignment between sleep-wake rhythms and the internal clock. Targeting the circadian disturbance with chronotherapeutic interventions may improve symptoms that most affect daily functioning and quality of life, ultimately preventing the downstream effects of OSA leading to disability. Supplementary exposure to bright light has beneficial effects on sleep, daytime vigilance and mood and it has been increasingly applied in a variety of sleep and neuropsychiatric conditions11 12. Our study is the first to use bright light therapy to improve sleep related functioning by reducing the residual daytime symptoms most strongly linked to disability in patients with sleep apnea. We will conduct a within-subjects crossover trial intervention to test the effects of BLT on and CPAP- resistant symptoms of sleepiness and depression and sleep, sleep-related functional impairment and quality of life in patients with OSA on CPAP therapy. We will also explore whether the circadian phase shifting moderates the degree of changes in daytime symptoms and functional impairment.