Background: Nonsuicidal self-injury (NSSI) is an identified prospective risk factor for suicidal thinking and attempts, but little is known about the proximal temporal relationship between NSSI and suicide. Information about possible mechanisms influencing engagement in NSSI and subsequent transitions to suicide is sparse, although different theories propose that self-criticism and a fearlessness of death of key contributors. Additionally, studies have suggested that contextual factors such as poor sleep may further exacerbate these proposed mechanisms to increase risk for suicide, but few studies have examined sleep’s proximal influence on risk. This study aimed to examine the concurrent (same day) mediational effect of NSSI urges on the relationship between self-criticism and fearlessness of death on next day suicide intent, as well as examine whether concurrent sleep problems moderate the effect. Methods: Participants included 119 young adults (Mean age = 19.85, SD = 1.23; 78.2% female; 91.3% White) with past month NSSI and suicide ideation. Participants completed daily surveys evaluating multiple risk factors including self-criticism, fearlessness about death, sleep problems, NSSI urges, and suicide intent across 28 days with a high compliance rate (87.1%). A multi-level moderated mediational path model was evaluated. Results: Results supported the full model. The indirect path from self-criticism to next day suicide intent through same day NSSI urge was significant (b = 0.043, p < .001) as was the indirect path from fearlessness about death (b = 0.014, p < .001). The interaction between concurrent sleep problems and NSSI urge was significant (b = 0.055, p < .01) supporting a moderation effect, although daily sleep problems did not have a direct effect on next day suicide intent (b = -0.011, p > .05). Conclusions: Disruptions in arousal systems, such as sleep, appear to exacerbate the risk-relationship of NSSI to next day suicide intent. Treatments that address sleep problems alongside psychological risk factors such as self-criticism and fearlessness of death may help to reduce short-term risk for suicide.