Recent research shows that sleep disturbances are linked to increased suicidal ideation. In the present longitudinal cohort study, we used subjective (ecological momentary assessment, EMA) and objective (actigraphy) measures to examine the effects of sleep parameters on next-day suicidal ideation. Further, we examined hopelessness as a mediator between insufficient sleep and increased suicidal ideation. Individuals with current suicidal ideation (N = 82) completed 21 days of EMA and actigraphy to estimate suicidal ideation, hopelessness and sleep parameters. Multilevel linear-mixed models were used to examine the effects of sleep parameters on next-day suicidal ideation, as well as for the mediating effect of hopelessness (in the morning) on the association between previous night's sleep and suicidal ideation levels the next day. Significant concordance existed between subjective and objective sleep measures, with moderate-to-large correlations (r = 0.44-0.58). Lower subjective sleep quality and efficiency, shorter total sleep time and increased time awake after sleep onset were significantly associated with increased next-day suicidal ideation (controlling for previous-day suicidal ideation). Actigraphy-measured sleep fragmentation was also a significant predictor of next-day ideation. Hopelessness mediated the effects of the subjective sleep parameters on suicidal ideation, but did not account for the association with sleep fragmentation. Therefore, individuals' psychological complaints (hopelessness, suicidal ideation) were better predicted by subjective sleep complaints than by objective sleep indices. Increased hopelessness following from perceived insufficient sleep appears an important explanatory factor when considering the link between sleep disturbances and suicidal ideation.