I found this information:
Antidepressants such as trazodone and mirtazapine, when used at low doses, are reliable sedating agents. Trazodone has sedating properties at doses much lower than those used for depression. Trazodone has been shown to decrease sleep latency while increasing delta sleep and having very little effect on REM sleep.11 Priapism is a serious but rare side effect associated with trazodone. Mirtazapine also has sedating properties, but that effect decreases with higher doses. Patients should be informed that mirtazapine may cause an increase in appetite, leading to weight gain. Mirtazapine should be used with caution in patients with hypertension. Overall, these antidepressants are viable options for patients with insomnia due to depression or insomnia due to antidepressant therapy. (here)
It's way down on towards the bottom of the page.
Also I take Trazadone for sleep issues. It was originally used as an anti-depressant but has been discovered to be more effective as a sleep med. I don't have any side effects from it and it has really helped me a lot.
On the other hand, I would suggest calling your doc and talking more to her about your concerns. Sometimes we have to be proactive when our gut is telling us something! I hope you get help for your insomnia. It's really hard to live your life when you can't sleep.
ETA I Googled "Mirtazapine for insomnia"