It may not be. Some states (not all) have laws which require at least an eight-hour break between full-time shifts. In addition, some federal regulations may apply as well, such as those which require a truck driver to take an 8 hour break for sleep in every 24 hour period. Check your local state law or call your local wage and hour division of the state department of labor.
Source: Epstein, Becker & Green, New York, June 16, 1998.