Now, it is the whole day that is thus to be spent [in holy exercises], i.e. the natural day. Not that people are bound to be in these exercises without intermission all the twenty-four hours; for God has not made the Sabbath to be a burden to man, but that we should continue God’s work as we do our own on other days, where we are allowed necessary rest and refreshment by sleep in the night.
Works of Thomas Boston, 2: 196.
Source: http://www.puritanboard.com/showthread.php/90810-Physical-Rest-on-the-Lord-s-Day, Comment #25