Oh, this will give many heartburn, but Kalitta Charters is in the club where one is continuously on call until getting a trip.
Typical scenario: you go available for duty on Monday at noon. At 10pm get dispatched to be wheels up at 11p to pick up freight in Greensboro, NC and drop off in Saltillo, Mexico. Although you had to answer the phone all day, they start your 135 "duty day" 30 minutes before your scheduled departure time. You take off pretty much on time and fly 1.5 to Greensboro. It's now 1230a. There is a freight delay; it takes 2.5 hours for the truck to show up. Let's say it takes another half hour to load freight, get your clearance, and set up for the next leg. You depart at 330a for Shreveport to refuel and a friendly spot to clear Customs out. It's 2 hours enroute, so 530a Eastern, but 430a Central -- which is Shreveport and Saltillo's time zone. While refueling dispatch calls. They've tried to set up your Saltillo arrival with a local handler, and learned Mexico is closed for the night. With the earlier freight delay, they couldn't keep the night guy working the tower in Saltillo. They tell you to hang out for a couple hours. It's 1.5 to Saltillo; don't arrive in Saltillo before 8a local.
You're tired because you've been up all day and now all night. Do you take a nap? For some that makes it worse. You do what works for you. If you nap you wake up in time to get your clearance, set up the next leg, and shake off the sleep inertia. You taxi out and are wheels up right at 630a local. You get to Saltillo right at 8a local and get put in holding because another freight carrier did the same thing from their starting point, but they beat you to MMIO by 3 minutes. The local tower is non-radar, so it's one-at-a-time for the approach/departure. Oh, you're flying round dials and first-generation GPS. (In fact you had to swap in the Mexico database card leaving SHV because those old things can't hold all of North America on one card.) There's nothing like clearing out the all-nighter mental cobwebs like figuring and hand-flying a hold entry in mountainous terrain.
It takes an hour to get unloaded and wait for your handler to file your flight plan back to Laredo, so you're off at 915a. It's maybe 0.7 back to LRD, and you clear Customs in. Usually easy empty. Taxi to the FBO, secure the airplane, finish the trip paperwork, go to the hotel. It's 1130a local -- only 12 hours of "duty." Yet you've been up all day and all night and now half the next day. You're hungry and sleepy. But the day has you perked up enough that you grab a Whataburger or some Taco Palenque before hitting the bed.
Now, I've been there when I've gotten called for my next trip right at the end of my required 10 hours of rest. And I've also waited, spring-loaded to go in 15 minutes, for up to 5 days. After a day or so, you get readjusted to day hours again so you can be awake when restaurants are open to serve. The longer you wait for the next trip, the more you wonder whether the call will come just after you've lathered up in the shower, just walked 5 blocks and ordered lunch, or perhaps have just entered REM stage sleep the next night. After your 10 hours of rest, the company considers you to be continuously legal for dispatch to begin a 14-hr duty day.