Despite a growing literature supporting the salutary effects of mindfulness meditation on emotion regulation, the underlying mechanisms linking neural and subjective changes occurring during the actual practice of meditation with emotion regulatory effects observed after meditation remains virtually unexplored. the current study sought to address this gap in knowledge by testing the hypothesis that adoption of internally-directed focused attention, indexed by increased alpha and theta spectral power, during brief open monitoring (oM) mindfulness meditation predicts reduced emotional reactivity, as measured by the late positive potential (Lpp). Results revealed that the OM meditation did not produce demonstrable differences in alpha and theta power but did increase self-reported sleepiness relative to controls. follow-up analyses showed that sleepiness uniquely moderated the effect of meditation on the LPP, such that less sleepiness during meditation, but not the control audio, corresponded to smaller Lpps to negative images. change in theta, but not alpha power, between meditation and rest was positively correlated with the Lpp even after controlling for sleepiness. Although the primary hypothesis was unsupported, the findings demonstrate that phenomenological and neural changes occurring during oM meditation may modulate its subsequent "off-the-cushion" effects on emotional reactivity. Originating from a 2,500-year old Buddhist contemplative tradition, mindfulness has received increased interest from people around the world. Although definitions of mindfulness vary across time and context 1 , one of the most cited contemporary definitions of mindfulness refers to the adoption of a nonelaborative, nonjudgmental awareness to present-moment experience 2,3. Driving its rising popularity, a rapidly growing body of research has shown that adoption and training of mindfulness, and possessing higher dispositional levels of mindfulness, are related to a wide array of benefits 2-5. A frequently cited benefit of mindfulness involves its salutary effects on emotion regulation 6,7 , a core selfregulatory ability involving modulation of the generation, experience, and expression of emotion 8. Despite lay and scientific consensus that mindfulness promotes healthy emotion regulation, little is known about how mindfulness confers its emotion regulatory effects. Research into this question is complicated by three factors. First, emotion regulation is conceptualized as a complex dynamic process that unfolds over time 9,10. Consequently, mechanistic investigations of mindfulness-based emotion regulation may strongly benefit from employing methodologies with temporal sensitivity. Second, mindfulness is a multi-faceted construct differentiable as a state, trait, and mind-body training modality 11,12. Moreover, mindfulness as mind-body training is itself highly varied, ranging from traditional sitting meditations to a panoply of didactic exercises and integrative interventions 13,14. This construct heterogeneity complicates operationa...