One must continue to toil or work regardless of how exhausted one is. A logical variant of the original phrase "no peace for the wicked." Parents of young children know all too well what it's like to have only a few hours of sleep and still have to get up at the crack of dawn—no rest for the weary, as my granny always said. There won't be time for a break, because we'll need to get going on the next phase of the project as soon as this one is done. There's no rest for the weary!