In Jain understanding, day and night are the natural rhythm of the world we live in and are explained in simple, practical terms:

• Day occurs when the Sun is above the horizon in a place, bringing light and activity.
• Night occurs when the Sun has moved away and the sky is dark, bringing rest.

This daily change is tied to the apparent motion of the Sun across the sky, which in modern terms is because the Earth spins on its axis. The same general idea is accepted in Jain lore when describing the ordinary, observable world.

If you’d like, I can explain how this day–night cycle fits into Jain cosmology (like how time is organized in kalachakra) in a simple way.