Here's why:

* Earth's Rotation: Earth spins on its axis, completing one rotation roughly every 24 hours. This rotation causes different parts of the Earth to face the Sun at different times.

* Sunlight: The side of the Earth facing the Sun receives direct sunlight, creating daylight. The side facing away from the Sun is in darkness, experiencing night.

It's important to note that Earth's tilt also plays a role in the length of days and nights throughout the year. This is why we have longer days in summer and shorter days in winter.