Trazodone as needed or constant?

My doctor prescribed me Trazodone for depression/sleeping issues. I'm petrified of medicinal side effects, so have been scared to take it. He said i can take it as needed, since its more to help me sleep. Does anyone know if thats true? that if i take it as needed to sleep (i drive for living so some days i can't risk being pill drowsy) its ok? Obviously he's my doctor and i should trust him but its still nice to get personal ''civilian'' opinions. Thanks!