The month is spent by Muslims fasting during the daylight hours from dawn to sunset. This light is most easily visible at night or under conditions of weak daylight. Ground reflected light also contributes to the daylight. The source of all daylight is the Sun. Their high brightness allows reading the display in bright daylight. This type of window allows for natural daylight and moonlight. Optimal vision occurs in the daylight hours. There is then no true daylight at the solar culmination, only civil twilight. Dromedaries are mainly active during daylight hours. This also creates a permanent daylight saving time effect. This is a response to both decreasing temperature and shorter hours of daylight. Many stations have no authorization to run at all outside of daylight hours. The higher levels had skylights carved through the mountain-side which provided daylight. The ultraviolet component of ordinary daylight is sufficient. As a result of exposure to daylight, all three were turned into stone.