In the Lochner v. New York case of 1905, the Supreme Court ruled that states could not impose limits on the number of hours that employees could work.
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A law passed in 1895 in the state of New York mandated that bakery employees could not work more than 10 hours a day and not more than 60 hours in a week. A bakery owner named Joseph Lochner filed suit against the state, claiming the law was unconstitutional. At the time, the Supreme Court decision was based on the idea that such laws violated an employee's "freedom of contract." The majority of justices saw such a right implicit in the due process clause of the 14th Amendment, thinking that if employees agreed to work a heavy number of hours it was their right to do so.
In the time since the Lochner case, the Supreme Court has gone in the other direction, allowing laws that impose reasonable restrictions on businesses. An example would be West Coast Hotel Co. v. Parrish (1937), which upheld the constitutionality of a minimum wage law passed in Washington state.
