The Earth's rotation causes darkness and day to alternate. Because we now know that the Earth's axis is tilted and, as a result, the equator does not always face the Sun directly, various parts of the planet experience uneven lengths of day and night—not exactly 12 hours of day and 12 hours of night all the time. Rather, there are two reasons why some regions experience more daylight than others: they are closer to the Sun, so get more direct sunlight, and they are further from the center of the Earth, so receive less gravitational force toward it mid-day.
Closer to the Sun means also closer to Earth, which means subject to its tides. So if you were to walk around in the dark, you would find that sea levels rise during the day and fall at night.
Further from the center of the Earth means also farther from its heat, which would cause them to cool down at night. But because the core of the Earth remains hot even when the surface becomes cold, this effect is much too small to account for half the night sky.
So what causes day to become shorter as the year progresses? It is called "seasonal variation". The reason for this is the Earth's orbit around the Sun. The distance between Earth and Sun varies throughout the year, causing days to become shorter as we move closer to solar maximum (which will be in 2013) and longer during solar minimum (which was in 2009).
The rotation of the Earth on its axis causes the variation between day and night. The day/night cycle would be considerably different, if not non-existent, if the Earth did not revolve as it does. In addition, the tilt of the Earth's axis and its route around the sun influence daylight hours. The result is that there are two distinct seasons, winter and summer, which repeat themselves each year.
As well as these external factors, there is also an internal one: our planet's core is solid but it is also liquid, which means that it can rotate without being destroyed. This core fluidity allows us to have both a rigid inner core and a soft outer shell. The inner core is mostly iron with some nickel and platinum; it is completely solid and has no connection to the outer shell or the rest of the planet. The outer shell is made up of a mixture of metal and rock called mantle. It lies deep under most of Europe and stretches all the way down to the center of the planet.
However, not all of the Earth's surface is equal - there are oceans and land masses. These differences in temperature and pressure cause different things to happen within the atmosphere and this affects what happens below on the surface. For example, water molecules are polar (they have a positive pole and a negative pole) which means that they attract each other. This is good for bonding together and forming compounds.
The true explanation for days and nights is found in the notion of the earth's rotation. At some moment in time, half of the earth's surface is illuminated by strong sunlight, forming day, and the other half is dark, forming night. The period of time during which this happens is called a day. The time it takes for the entire earth to rotate once is called a day.
As you travel around the world, you will see that the sun rises in the east and sets in the west. This is because the eastern part of the earth is now in darkness, while the western part is still under the sun's influence. At some point in time, the entire east will be under the sun's glow and then later the west. This is why there are 24 hours in a day. If the earth did not rotate, then at some point in time, all points on the planet would be exposed to the sun at the same time.
This rotating frame of reference creates the need for sleep. We need sleep because our brains produce certain chemicals that help us fall asleep. These same chemicals are released when we wake up, so we need to sleep again to let them out. If we did not sleep each night, these chemicals would build up and cause problems with cognition and mood.
The durations of days and nights vary depending on where you are on Earth and the season. In the summer, when the Earth is farthest from the Sun, days are long and cold while in the winter, when the Earth is closest to the Sun, nights are long and cold.
The length of a day is based on when the Earth reaches its maximum distance from the Sun. At this point, the North Pole is tilted away from the Sun, while the South Pole is facing it. Daily sunlight exposure increases the amount of energy that reaches the surface, which allows for life to exist during these periods. As the Earth moves closer to the Sun, days get shorter and colder until spring when the Earth is again furthest from the Sun and days become longer and warmer.
The duration of a night is based on when the Earth is nearest to the Sun. At this point, the North Pole is facing the Sun, while the South Pole is away from it. Daily sunlight exposure decreases the amount of energy that reaches the surface, which allows for life to exist during these periods. As the Earth moves farther from the Sun, nights get longer and cooler until fall when the Earth is again nearest to the Sun and nights are short once more.