Why have i been prescribed an antidepressant for insomnia?

Maybe sedating?: You should ask your treating physician why he or she prescribed this. In the absence of this information, insomnia may be a symptom of depression, and the thinking is that treatment of the depression will resolve the insomnia. Another possibility is that you were prescribed a sedating antidepressant for its sedative properties, in an attempt to help you sleep.

Off label : When Trazadone, aka Desyryl, was invented in 1980's, it was discovered to have significant side effects of drowsiness. Doctors, who are allowed to use medications "off label", fashionably use Trazadone as a "sleeping pill". Interestingly, the FDA-approved use of Trazadone for depression & advised a three-time-a-day administration schedule! So much for safe driving.