Trazodone is one of the most commonly prescribed off-label medications for insomnia in the United States, despite the FDA never having reviewed or approved it specifically for that use. The practice grew out of decades of clinical observation: the same antihistamine-driven sedation that makes trazodone drowsy-inducing at antidepressant doses is achievable at much lower doses — often 25 to 100 mg — without needing a full antidepressant dose.
Prescribers sometimes favor it over benzodiazepines or Z-drugs (like zolpidem) because it is not a controlled substance and carries a different dependence profile, or because a patient also has coexisting depression or anxiety it might help with.
That said, off-label doesn't mean untested — it means the specific FDA review-and-approval process for insomnia specifically hasn't happened, even though the underlying safety and interaction data for trazodone itself is extensive.