Background: Although sleep is an important risk factor for suicidal behavior, research has yet to examine the association between sleep problems and suicidality across the course of inpatient treatment. This study examined the relationship among sleep-related symptoms and suicidal ideation across inpatient treatment. Aims: To examine whether poor sleep at admission longitudinally predicts less improvement in suicidal ideation over the course of treatment. Further, to examine whether suicidal ideation is reduced in patients whose sleep does not improve. Method: The study utilized the Beck Depression Inventory (BDI)-II, which contains items measuring depressive symptoms, sleep-related symptoms, and suicidal ideation. The study sample consisted of 1,529 adult psychiatric inpatients. Patients were assessed at admission, biweekly, and at treatment termination. Results: Admission fatigue, loss of energy, and change in sleep pattern were associated with higher levels of suicidal ideation at admission and discharge. Fatigue at admission predicted suicidal ideation at termination independent of admission depression and suicidal ideation. Individuals whose sleep did not improve over the course of treatment had significantly higher suicidal ideation scores at termination relative to those whose sleep symptoms improved, after controlling for sleep, depression, and suicidal ideation scores at admission. Conclusion: These findings suggest that persistence of sleep-related symptoms warrants clinical attention in the treatment of suicidal patients.