The cause of day and night on earth is caused by the rotation of the earth and because it tilts on it's axis as it turns.
The earth is spinning at thousands of miles per hour even though we can't feel it because of gravity.
As the earth spins it tilts until it gets away from the sun which causes the earth to get darker and darker and then as morning comes again the earth begins tilting more closer to the sun.
As the earth rotates and tilts closer to the sun the daylight comes out and the earth gets lighter and lighter where we can see and the dark goes away.
As the day progresses the earth continues to spin and tilts back away from the sun again which causes it to get dark on earth again.
And it repeats this process for millions of years day in and day out.
Another thing is that the daylight hours are affected by the tilt of the Earth's axis and its path around the sun.