A growing body of literature supports the notion that sleep disturbances, including insomnia, are related to suicidality. However, the mechanism through which insomnia correlates with suicide risk is unclear. The primary goal of the present research was to determine whether hopelessness, a robust predictor of suicidality, mediates the relation between insomnia and suicidal ideation (SI). Additionally, analyses were conducted to determine which demographic, health, sleep, and daytime functioning variables best predict hopelessness. Finally, this research will address gaps in the literature by determining which types of insomnia best predict suicide risk (i.e., hopelessness and SI), and whether a complaint of insomnia, poor sleep (as defined by quantitative criteria), or the combination of these factors best predicts suicide risk. The present study used an existing data set consisting of self-report data from a community-dwelling epidemiological sample. Participants (n = 766) completed a Health Survey, two weeks of daily sleep diaries, and five measures of daytime functioning, including the Beck Depression Inventory (BDI). BDI item 2 was used to assess hopelessness, and BDI item 9 was used to assess SI. Criteria from the Diagnostic and statistical manual of mental disorders (4th ed., text rev.) as well as quantitative criteria were used to identify participants with insomnia (n = 135). The analyses revealed that hopelessness is a significant mediator of the relation between insomnia and SI (Percentile 95% CI [0.24, 0.71]). Additionally, stepwise logistic regression revealed that, of a large pool of candidate variables, depression, anxiety, mean sleep efficiency, and intra-individual variability in sleep quality ratings are the best predictors of hopelessness. Finally, stepwise logistic regression revealed that a complaint of insomnia is a better predictor of suicide risk than quantitatively-defined poor sleep or the combination of these factors. Recommendations for future research include determining whether cognitive behavioral therapy for insomnia decreases hopelessness, which could ultimately decrease suicide risk. Additionally, the present research suggests the need for clinicians to routinely screen clients who report insomnia for hopelessness and SI.