Crazybird,
The times for cargo flights vary with the purpose of the flights. I operate internationally, and our airplanes keep moving. The time of day the flight takes place really depends on when the last flight arrived, how long removing cargo and fueling takes place, arrival times for new cargo, delays that occurred before the last leg, and so on.
Our airplanes will typically arrive somewhere, change crews and get fuel, and be gone again in two hours. Even a short leg can make a big diference in the next flight's departure time as it crosses time zones. Often one flight I do will leave in the night, while the next leaves in the day. Sometimes we will take off just prior to night fall, and the night will pass away within a few hours and become day light again as we fly east. Sometimes we even depart one day,and arrive somewhere else the day before...because of time zones, direction of flight, the international date line, and the length of that leg.
Some flights are conducted at night because freight is collected in the daytime, and distributed in the daytime. If you send a package during the day, it will be flown overnight to the destination, and delivered to the recipient in the daytime.
I see you're in Bahrain. I generally arrive there in the dark and leave again around the dark. Part of the reason for that is because it is a little cooler at night, and airplane performance is predicated on temperature, as well as weight. However, Bahrain is a crowded place and airplanes aren't allowed to remain there for long periods...waiting for the next night. In fact, sometimes when airplanes arrive, they're offloaded, then flown to another airport to make room, until the next load of cargo is ready to depart. When, then, depends on a number of factors...including arrivals scheduled at other locations, the airport congestion, and delays that can range from getting fuel to packaging the cargo.