Why does it get darker in winter?

The seasons affect the amount of sunlight the Earth receives due to its axial tilt. During the winter months, when a hemisphere is tilted away from the sun it receives less sunlight and the nights are longer. This also reduces the temperature due to less solar radiation reaching the ground.

While most areas experience longer nights during the winter, areas near the poles can actually experience polar night: a night time that lasts more than 24 hours. This happens due to some locations being tilted so far away from the sun, that the Earth blocks all light for an extended period. The city of Kiruna, Sweden is known for periods of extended darkness that can last up to 28 days.