Prolonged grief disorder, characterized by severe, persistent and disabling grief, has recently been added to the DSM-5-TR and ICD-11. Treatment for prolonged grief symptoms shows limited effectiveness. It has been suggested that prolonged grief symptoms exacerbate insomnia symptoms, whereas insomnia symptoms, in turn, may fuel prolonged grief symptoms. To help clarify if treating sleep disturbances may be a viable treatment option for prolonged grief disorder, we examined the proposed reciprocal relationship between symptoms of prolonged grief and insomnia. On three time points across six-month intervals, 343 bereaved adults (88% female) completed questionnaires to assess prolonged grief, depression, and insomnia symptoms. We applied random intercept cross-lagged panel models (RICLPMs) to assess reciprocal within-person effects between prolonged grief and insomnia symptoms and, as a secondary aim, between depression and insomnia symptoms. Changes in insomnia symptoms predicted changes in prolonged grief symptoms but not vice versa. Additionally, changes in depression and insomnia symptoms showed a reciprocal relationship. Our results suggest that targeting insomnia symptoms after bereavement is a viable option for improving current treatments for prolonged grief disorder.