There must be a maximum amount of performance that people are capable of, given rest and reward. To work people too long is to waste their potential.  Human beings require downtime in the form of sleep and rest in order to maintain their peak functioning. Long working hours cut into this rest and sleep time and therefore reduce their effectiveness as workers. A cap on the amount of work that people do per week allows for proper rest periods. Tired workers are prone to making mistakes, one of the mistakes they can make is to think they can skip necessary sleep with no ill effects. "While some people may like to believe that they can train their bodies to not require as much sleep as they once did this belief is false"1  A mandatory cap on the hours they work removes the decision from them and avoids this problem.  1 Sarah Ledoux – “The effects of sleep deprivation on brain and behaviour” Biology 202 Bryn Mawr College 01/03/2008