Daily life stressors and negative emotional experiences predict poor physical and psychological health. The stress response of the hypothalamic-pituitary-adrenal axis is a primary biological system through which stressful experiences impact health and well-being across development. Individuals differ in their capacity for self-regulation and utilize various coping strategies in response to stress. Everyday experiences and emotions are highly variable during adolescence, a time during which self-regulatory abilities may become particularly important for adapting to shifting social contexts. Many adolescents in the U.S. enter college after high school, a context characterized by new opportunities and challenges for self-regulation. Guided by biopsychosocial and daily process approaches, the current study explored everyday stress and negative affect (NA), cortisol reactivity, and self-regulation assessed at the momentary, daily, and trait level among a racially/ethnically and socioeconomically diverse sample of first-year college students (N = 71; Mage = 18.85; 23% male; 52% non-Hispanic White) who completed a modified ecological momentary assessment. It was expected that within-person increases in momentary stress level or NA would be associated with cortisol reactivity assessed in college students' naturalistic settings. It was predicted that these within-person associations would differ based on engagement coping responses assessed via momentary diary reports, by the range of engagement coping responses assessed via diary reports at the end of the day, and by higher trait levels of self-regulation assessed via standard self-report questionnaire. Within-person increases in momentary stress level were significantly associated with momentary elevations in cortisol only during moments characterized by greater than usual engagement coping efforts (i.e., within-person

increases). At a different level of analysis, within-person increases in momentary stress level were significantly associated with increases in cortisol only for those with low trait levels of coping efficacy and engagement coping. On average, within-person increases in momentary NA were significantly associated with cortisol reactivity. Tests of moderation revealed this momentary association was only significant for those with low trait levels of support-seeking coping.