Severe, persistent and disabling grief occurs among a sizable minority experiencing bereavement, with diagnostic manuals newly including complicated grief (CG) disorders. Sleep disturbances/disorders have been established as worsening affective and stress-related conditions. However, the role of sleep difficulties in bereavement and CG has not received similar scientific attention. We therefore conducted a systematic review with narrative syntheses on this topic to clarify the role of sleep in bereavement (PROSPERO: CRD42018093145). We searched PubMed, Web of Science and PsychInfo for peer-reviewed English-language articles including (at least one) bereaved sample and sleep disturbance measure. We identified 85 articles on 12.294 participants. We answered seven pre-defined research questions demonstrating: high prevalence of sleep disturbances in bereavement; positive associations of grief intensity with sleep difficulties; preliminary indications of risk factors of post-loss sleep disturbance; higher prevalence of sleep disturbances in CG, enhanced by psychiatric comorbidity (i.e., depression); and initial evidence of causal relationships between (complicated) grief and sleep. Grief therapy partly improves sleep difficulties, yet no intervention studies have specifically targeted sleep problems in bereaved persons. Causal relationships between sleep and grief require further examination in intensive longitudinal investigations, including randomized trials, thereby clarifying whether treating sleep problems enhances CG treatment effects.