This study examines the role of rapid eye movement (REM) sleep in emotional reactivity in young women. Sleep plays a critical role in emotional regulation, with REM sleep specifically involved in affective processing. Although prior research has examined hormonal influences on sleep in reproductive-aged women, the interaction between REM sleep microarchitecture and emotional reactivity across menstrual phases remains largely unknown. Previous work indicates that the luteal phase, characterized by elevated progesterone, is associated with increased negative affect and heightened emotional reactivity in naturally cycling women. Therefore, this study investigates whether emotional reactivity is heightened during the luteal phase and whether REM sleep characteristics predict individual differences in emotional outcomes within and across phases. Using a within-subjects design, participants complete two overnight laboratory sleep sessions, one in the follicular phase and one in the luteal phase, while undergoing polysomnography (PSG). Emotional reactivity is assessed before and after sleep using an emotional memory picture task. REM sleep is quantified using REM duration and percentage to determine whether menstrual phase-dependent differences in REM sleep are associated with changes in emotional response. It is hypothesized that REM sleep duration will be reduced and emotional reactivity will be greater during the luteal phase compared to the follicular phase. Additionally, REM sleep percentage is expected to be associated with emotional reactivity in both phases. By examining sleep physiology alongside emotional outcomes, this research aims to clarify how ovarian hormones influence sleep-dependent emotional regulation and contribute to menstrual phase-related vulnerability to emotional dysregulation.