Day length refers to the duration of time from sunrise to sunset. It varies throughout the year due to the tilt of the Earth's axis and its orbit around the Sun. In general, days are longer in the summer and shorter in the winter. The concept of day length is important for various natural processes, including photosynthesis in plants and the behavior of animals. Different regions on Earth experience varying day lengths, especially near the poles, where during certain times of the year, the sun may not set at all or may not rise at all.