Not sure about US laws but in Europe there's mendatory rest periods for drivers. Drive n hours, sleep/rest m hours where m is relatively large. For an autonomous vehicle that time is reduced to zero (I'd assume)
Mandatory rest exists in the US. But i assume the guy watching over the self driving car, should follow those laws.
That assumes that it needs supervision and the driver isn't just there for the first/last mile.
Once you don't need the driver you don't have to worry about sleeping - the truck can drive 24/7 without breaks
Truckers operate on the 14/11 rule currently. 14 hours from when you wake up, 11 hours from the start of driving. If they can throw that out a tractor trailer can log twice the hours in a 24 hour day than current regulations allow.
Maybe the mandatory resting periods for drivers will be less strict when they're driving a self-driving truck.