Dan is quite correct. However, and I’m sure you know this, the 24 hr periods which he refers to are not midnight to midnight as a lot of people believe, but are the 24 hrs which commences from whenever you start your shift.
I don’t know how your work is organised, but a lot of drivers like to try to keep a 15 hr available till the last duty period of the week, just in case the wife has a big juicy steak on the grill on the Friday night, and then you run out of spread over and have to overnight in a layby with a can of Monster and a Ginsters lung and scrotum pasty.
Meanwhile, her indoors, having bought and cooked the steak, doesn’t want to waste it, so has to find someone to share it with.
I think this topic has been done to death already but hey ho!!