Objective To elucidate processes contributing to continuing self-harm in youth at very high risk for suicide, focusing on sleep disturbance, a putative warning sign of imminent suicide risk. Method 101 youth (ages 12–18) selected for high risk of suicide/suicide attempts based on suicidal episodes plus repeated self-harm (suicide attempts and/or nonsuicidal self-injury [NSSI]). Youth were assessed at baseline, 6-, and 12-month follow-ups on measures of self-harm, suicidality, sleep, and depression. Results Youth showed high rates of baseline sleep disturbance: 81.2% scored in the clinical range on the Pittsburgh Sleep Quality Index (PSQI); 81.2% reported an evening (night owl) circadian preference. PSQI score was associated with elevated levels of self-harm (suicide attempts and NSSI) contemporaneously and predicted future self-harm within 30 days. Rates of self-harm were high during follow-up: 45.0% and 33.7% at 6 and 12 months, respectively. Conclusions Results underscore the need to move beyond an acute treatment model to prevent recurrent and potentially deadly self-harm, the importance of clarifying mechanisms contributing to elevated suicide/self-harm risk, and the potential promise of engaging sleep as a therapeutic target for optimizing treatment and elucidating mechanistic processes.