What does it mean when polar nights have twilight?
Civil twilight: the sun is between 0 and 6 degrees below the horizon
Nautical twilight: the sun is between 6 and 12 degrees below the horizon
Astronomical twilight: the sun is between 12 and 18 degrees below the horizon

I believe if the sun is lower than 18 degrees below the horizon, all sun rays can no longer reach your location. Hence it is completely dark.