You raise a very interesting point. Engels said (I can't find the quote at the moment) that the time when the employee is away from work, i.e not selling his labour-power, he is useful to the employer, as the worker repairs himself and maintains himself with the wage given to him, and recharges his faculties by rest and sleep. In this way, the employee is never off the clock, he is always working for that employer in some form, readying himself for more work.

This is evident; the employer pays for the upkeep and maintenance of a machine, and in the same way he pays for the upkeep and maintenance of the worker in the form of a wage. The worker is a self-fixing machine which sells a commodity which is able to not only transfer but create value.

Hourly contractors should be exempt.