Actually, it depends on where you are. FedEx (as I'm sure UPS and Airborne) doesn't fly their NB AC all that much other than nights, but it would be hard to find a FedEx widebody not flying all day and all night. In fact, that causes some problems for the "airline" when the schedule is wound so tight.
For instance, the midnight (Actually it's about 0300) flight from MEM to...say LAX will turn to a day flight to...the IND day sort (for all the 2 day freight) then that AC would (in theory...not the same EXACT AC--just same type AC) turn back to the west coast (LAX, SEA, PDX etc etc that would in turn fly right back to the MEM, OAK, AFW, IND or EWR night sort...
That happens for most of the larger west coast cities, and includes all the major hubs FedEx operates from.
To answer another part of your question,,,FedEx tries NOT to fly everything to MEM.
Generally speaking, if a particular city has enough freight for only one hub, that hub will always be Memphis. Then depending on where the city is, will determine what "secondary" hub an additional AC will operate to. For the very large cities on both coasts, they may have widebody AC going to three or more of the hubs FedEx operates to keep the volume out of MEM. So for instance, if you ship something from LAX to...CAE, it will come to MEM, but if you ship something from LAX to FLL, it will likely move LAX-AFW (FedEx's Dallas hub), AFW-FLL. Remember the old saying...You can only put five pounds of...mud in a five pound bag. As it stands now, MEM gets over 135 flights a night from across the country in mainly a 2.5 hour window. As I understand it, UPS is the same in SDF.
The main freight companies don't as a regular plan use the pax carriers...they use their own dedicated fleet. More control, more flexibility.
I hope this helps.