AFAIK mirtazapine shouldn't cause habituation the way actual "sleeping pills" or benzodiazepines do. That's one of the reasons it may be preferable as a sleeping aid, especially in the longer term.

Anecdotally, when I took mirtazapine for sleeping problems, it did sometimes seem to have a stronger effect the first time I took it after not using it for a while. After that the effect stayed stable. Overall it shouldn't cause habituation, and my doctor said as much.

Of course trust your doctor and not strangers on the internet, though.