People have a limited capacity of work.
Although we all have different limits, the employee has a certain capacity for work within a certain frame of time. This frame is expressed at different resolutions, just like the workload. For example, an employee can’t work more than 3 consecutive hours before getting a break. That same employee can’t work more than 7 consecutive days before getting a day off. It is the same rest constraint, but expressed at a different resolution (hours vs. days).