Abstract Introduction Obstructive sleep apnea (OSA) is characterized by recurrent airway obstruction causing intermittent hypoxia, arousal from sleep, and autonomic instability, and is associated with a variety of long-term health sequala. The fragmented sleep resulting from these respiratory events leads to disruption in normal sleep architecture and progression to deeper stages of sleep, a potential mediator for some of the functional and neurocognitive impairments seen in this population. Similarly, psychoactive substance use (i.e., nicotine, alcohol, and caffeine) impacts sleep architecture in healthy individuals, but their effects in those with OSA have not been well described. We aimed to describe the association between psychoactive substance use and sleep characteristics and daytime symptoms in individuals with untreated OSA. Methods We performed a secondary, cross-sectional analysis of The Apnea Positive Pressure Long-term Efficacy Study (APPLES). Exposures included current smoking, alcohol, and caffeine use in individuals with untreated OSA. Outcome domains included subjective and objective sleep characteristics, daytime symptoms, and co-morbid conditions. Linear or logistic regression assessed the association between substance use and each domain (e.g., self-reported sleep duration, total polysomnographic sleep time, sleepiness, and anxiety). Results Of the 919 individuals with untreated OSA, 116 (12.6%) were current cigarette smokers, 585 (63.7%) were moderate or heavy alcohol users, and 769 (83.7%) were moderate or heavy caffeine users. Current smokers exhibited lower sleep duration (0.3 hours) and longer sleep latency (5 minutes) compared with non-smokers (all p-values< 0.05). REM sleep increased in those with moderate and heavy alcohol use (2.5 and 5% of total sleep time respectively), and moderate caffeine use (2%, all p-values < 0.05)). The combined smoker plus caffeine group exhibited shorter sleep duration (0.4 hours) and higher risk for chronic pain [Odds Ratio (95%CI) = 4.83 (1.57, 14.9). . Conclusion Both psychoactive substances and untreated OSA impact normal sleep architecture, their cumulative effect may have clinical implications in this subgroup. Stimulating or sedating substances may be used in OSA patients to mitigate daytime and nighttime symptoms respectively. Further investigation into the effects substances have on this population may present opportunities to better understand disease mechanisms and increase the effectiveness of treatment in OSA Support (if any)