I think insomnia is less likely than the reverse.

In antidepressant doses, the side effect profile is typically different than in the lower doses used for PN.

It can take up to a month to see effects.

When I tried amitriptyline (the parent compound) last winter I had pain relief within 3 days. But that wore off in the low dose I was taking after about 10 days. Because it made my blood sugar go up, I discontinued it. No sense in raising the dose etc, for me.

Nortriptyline is the active metabolite of amitriptyline (Elavil). So their actions are very similar.