...An additional caution is that it is a violation of FAR 91.13 for a certificate holder or a flight crewmember to operate an aircraft in a careless or reckless manner so as to endanger the life or property of another. Lack of rest of the pilot is certainly a circumstance which could endanger others, and it is not necessary that the situation devolve into actual endangerment for there to be a violation of FAR 91.13. A certificate holder who uses a crewmember with knowledge of his or her lack of rest may be equally culpable along with the flight crewmember...
Bodlak Interpretation