RRB Group D 1-Dec-2013 Shift 1 Solved Paper

The polar night occurs in the northern-most and southern-most regions of the Earth when the night lasts for more than 24 hours. This occurs only inside the polar circles.
The opposite phenomenon, the polar day, or midnight sun, occurs when the sun stays above the horizon for more than 24 hours.
"Night" is understood as the centre of the sun being below a free horizon. It is because of the inclination of the earth towards its orbital plane.