Sleep disturbance is associated with elevated suicidal ideation and negative affect. To date, however, no study has investigated the temporal relationship between sleep and suicidality among those diagnosed with borderline personality disorder (BPD). This preregistered (https://osf.io/4vugk) study tested whether nightly sleep (self-reported sleep duration, sleep onset latency, and subjective sleep quality) represents a (within-person) short-term risk factor for affective dysregulation and increases in suicide risk from day-to-day, as well as whether between-person differences in sleep, negative affect, and suicidality were associated. We used a 21-day ecological momentary assessment protocol in a sample of 153 people diagnosed with BPD, 105 of which had a history of serious suicide attempts, and 52 healthy controls (N = 4076 days). We found a within-person association between worse subjective sleep quality and greater next-day negative affect. At the between-person level, we found positive relationships between sleep latency and suicidal ideation, and a negative association between subjective sleep quality and negative affect. BPD severity did not significantly moderate the strength of any within-person associations, although BPD was positively associated with average levels of suicidal ideation, sleep latency, and negative affect, and negatively related to subjective sleep quality. These findings suggest that the association of sleep with suicidal ideation and BPD exists largely at the between-persons rather than the within-person level. Disturbed sleep, therefore, seems to largely coincide, rather than specifically contribute to, the exacerbation of suicidal crises in BPD. (PsycInfo Database Record (c) 2022 APA, all rights reserved).