The risk of suicide is a significant concern in patients with Borderline Personality Disorder (BPD): 70% of those diagnosed with BPD have attempted, and 5–10% completed suicide (Black, Blum et al. 2004; Zanarini, Frankenburg et al. 2005; McGirr, Paris et al. 2007). Although research suggests that the intricate association of BPD and suicidal ideation is likely potentiated or enhanced in individuals reporting sleep disturbances (Winsper & Tang, 2014; Winsper et al., 2017), the temporal linkage of sleep and suicidal ideation in BPD has received limited clinical and research attention (Winsper & Tang, 2014). In this study, we aim to address this gap by examining the influence of nightly sleep on next day suicidal ideation in a diverse BPD patient sample varying in degree of lethalithy of past suicidal attempts. Suicidal ideation, suicidal attempts, and even death by suicide have been linked to variety of indicators of poor sleep health (see Buysee, 2014), such as insomnia, poor subjective sleep quality (Bromundt et al., 2013; Harty et al., 2010; Oltmanns, Weinstein, & Oltmanns, 2014; Sansone, Edwards, & Forbis, 2010; Selby, 2013), short self-reported sleep duration, and low sleep efficiency (Hafizi, 2013). Much of this past research has adopted cross-sectional designs to investigate the association between poor sleep and suicidal ideation (Bernert et al., 2015). While providing an informative starting point, these cross-sectional designs fail to characterize the temporal aspects of the relationship between sleep and suicidality, both of which fluctuate over time and may influence each other (Ben-Zeev et al., 2012; Kleiman et al., 2017; Lemola et al. 2013; Vallieres et al. 2005). To date, four longitudinal studies have examined the possible bidirectional associations between sleep and suicidal ideation, yet none of them in BPD samples in particular. Ribeiro et al. (2012) found that greater insomnia symptoms predicted greater suicidal ideation one month later in a sample of young military recruits, but not vice versa. These findings were replicated by Zuromski et al. (2017) in an online sample of individuals endorsing lifetime experience of suicidal behavior. Additionally, a recent one week-long ecological momentary assessment study reported that subjective and actigraphic sleep duration as well as subjective sleep quality predicted next day ideation (Littlewood et al., 2018). Again, suicidal ideation did not predict next night sleep. Finally, a unidirectional relationship was reported between nightmares significantly predicting post-sleep self-harming thoughts and behaviors in a sample of university students (Hochard et al. 2015). This relation was partially accounted for by negative affect upon awakening, suggesting that the affective consequences of poor sleep may be one reason sleep influence suicidal behaviors Increasing evidence suggests that sleep problems chart the progression of symptom severity and suicidality in BPD via self-perpetuating circles of exacerbated emotional dysregulation (Balestrieri et al., 2006; Sjostrom, Waern, & Hetta, 2009; Wojnar et al., 2009). Poor sleep is frequently linked dysregulated affect, particularly higher negative affect and increased emotional reactivity (Leger et al., 2019; Palmer & Alfano, 2017; Zohar et al., 2005). Emotion dysregulation likely impacts the ability to fall asleep and disrupt sleep quality (Konjarski, Murray, Lee, & Jackson, 2018), perpetuating a destructive cycle between sleep problems and emotion dysregulation over time. This suggests that sleep disturbances may contribute to affective dysregulation observed in BPD. Reciprocally, affective dysregulation that characterizes BPD may also contribute to insomnia over time (Fitzpatrick et al., 2020). In longitudinal studies affective instability and the persistence of negative affectivity have been theoretically and empirically related to the risk of suicidal behavior and attempts in BPD (Linehan, 1993; Wedig et al., 2012; Yen et al., 2004). Moreover, both mean levels and variability in negative affect uniquely characterize BPD and connect BPD to outcomes such as wellbeing (Dejonckheere et al., 2019; Houben & Kuppens, 2020). Finally, research based on intensive longitudinal sampling approaches suggests that the association between negative affective states, and the severity of suicidal ideation is strongly pronounced in BPD (Links et al., 2007; Mou et al., 2018). Altogether, poor sleep may lead to dysregulated emotions (e.g., increased mean levels and variability in negative affect), which in turn may amplify suicidal ideation in people with BPD. Although sleep disturbance has been robustly found to contribute to elevated suicidal ideation in clinical samples, and that this relationship is likely accounted for by daytime affective dysregulation, no study has investigated the temporal relationship between sleep and suicidality in BPD. Therefore, the current study seeks to follow a recent call for more methodologically rigorous studies of this relationship (Bernert et al., 2015), and investigates the temporal relationship between sleep problems, emotion dysregulation, and suicidal ideation using a 21-day ecological momentary assessment protocol in a sample of BPD patients and healthy control participants. Because past suicide attempts represent the most reliable predictors of future suicide attempts, this sampling approach increases the likelihood of participants reporting suicidal ideation during the study period. Moreover, the design of this study allows us to improve limitations in the suicide literature by utilizing a short follow-up period and focusing on sleep as a less studied risk factor for suicidal ideation in BPD.