Thomas Boston, Works, 3:201:
We have a careful enemy that watcheth all advantages and opportunities against us. “As a roaring lion, he walketh about seeking whom he may devour.” While we sleep, the devil wakes; while we stand still, he goes about us; while we fear nothing, he takes the advantageous occasion to make his attack. Ah! shall not the diligence of our malicious enemy, rouse us to look about us and be upon our guard. Our souls are a prey, which he thinks well worth his pains to catch, and do not we reckon it worth all our pains to keep them?
Source: http://www.puritanboard.com/f25/while-we-sleep-devil-wakes-83726/, Comment #1 (Thank You, Reverend Winzer!)