As the Earth spins on its axis, the side facing the sun has day while the other side has night. As the earth continues to spin, the shaded side gets light.
The Earth takes about 24 hours to make a complete rotation on the axis causing day and night. Our clocks are based on this principle and now, the atomic clocks are able to measure the exact time, even if the earth speeds up or slows down as it spins.
Below Day and Night info from Wikipedia (Read More):-
A day (symbol d) is a unit of time equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI.[1] The SI unit of time is the second.
The word 'day' can also refer to the (roughly) half of the day that is not night, also known as 'daytime'. Both refer to a length of time. Within these meanings, several definitions can be distinguished. 'Day' may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?".
Night or nighttime is the period of time when the sun is below the horizon. The opposite of night is day (or "daytime" to distinguish it from "day" as used for a 24-hour period). Time of day varies based on factors such as season, latitude, longitude and timezone.
http://en.wikipedia.org/wiki/Day
http://en.wikipedia.org/wiki/Night
