I have anxiety disorder & insomnia .i very sleepy in the day but when night comes i can't sleep even with mirtazapine , what cause this ?how to solve?

How is your sleep: hygiene? Retire & rise same time each day. Keep bedroom dark & cool & use only for sex & sleep. Turn off tv. No naps. Exercise regularly but not in late evening. No caffeine for 6 hrs & no alcohol or tobacco w/i 2 hrs of sleep. Keep dinner moderate sized & finish at least 2 to 3 hrs before sleep. Can try warm milk, Chamomile tea or melatonin along w light reading, warm bath or lavender scent.