Project Summary Sleep deficiency, given its impairing effects on emotion, cognition, and decision-making, is likely a key risk factor contributing to non-adherence to antiretroviral (ART) medications in older HIV-positive gay and bisexual men (GBM). However, research on sleep deficiency’s effect on ART adherence and on self-efficacy regarding adherence is very limited, has been mostly cross-sectional in nature, and has not focused on older adults. We are submitting this application in response to PAR-17-320, with the aim of conducting novel, hypothesis-driven exploratory research into sleep deficiency among older HIV-positive GBM within a daily diary study design. We propose to recruit and enroll a racially-diverse sample of 100 HIV-positive GBM, aged 50 or older, who report suboptimal adherence at baseline. Gathering 21 days of twice-daily diary data and wristwatch actigraphy data for objective sleep indicators, we will work to achieve three primary aims: SPECIFIC AIM 1: To test the direct effect of day-level sleep quality ART adherence on a given day. SPECIFIC AIM 2: To examine whether a potential mechanism by which day-level sleep quality affects ART adherence is through its direct effect on day-level self-efficacy for adherence. SPECIFIC AIM 3: To gather qualitative feedback from participants about sleep, ART adherence, barriers to improving both, and recommendations for how sleep health interventions could be tailored to this population. By exploring the role of sleep deficiency in ART adherence among older GBM, as well as gleaning qualitative feedback from participants in the follow-up assessment regarding sleep health, perceived barriers, and intervention possibilities, the proposed project has high potential to improve adherence interventions aimed at achieving viral suppression and improved wellbeing among this population, with potential future applications to other groups.