I have a portable radio which serves for FM / MW / SW bands. At nights I can listen to 4-6 international channels from different countries on MW. However, I am unable to tune them at daytime. Is it about my location as a result of earth's spin or there are other reasons that I don't know? Shutting down the radio station at nights comes to my mind but all the stations are from different (meridians) countries.
During the day, the ionosphere is strengthened by direct exposure to the solar wind, making it thicker and lower relative to the Earth's surface. This prevents any sort of skip propagation at MW frequencies, limiting MW propagation to ground wave only, which only covers a limited range from the transmitter.
At night, the ionosphere is higher and more diffuse, allowing skip propagation to much longer distances.
These effects have been known for a long time, which is why most MW stations are required to reduce power at night in order to reduce interference with distant stations on the same frequency. Only designated "clear-channel" stations are permitted to continue to operate at full power.