And nature must obey necessity; / Which we will niggard with a little rest. In North's Plutarch, Shakespeare's primary source, it is written that "Brutus . . . slept very litle, both for that his dyet was moderate, as also bicause he was continually occupied. He never slept in the day tyme, and in the night no lenger, then the tyme he was driven to be alone, and when every bodye els tooke their rest."1
1Plutarch, Plutarch's Lives of the Noble Grecians and Romans Englished by Sir Thomas North, trans. Sir Thomas North, vol. 6 (1579. London: David Nutt, 1896) 217.