Doubt may be fostered by sleep deprivation. If you keep burning the candle at both ends, sooner or later you will indulge in more and more mean cynicism—and the line between cynicism and doubt is a very thin one. Of course, different individuals require different numbers of hours of sleep: moreover, some cope with a bit of tiredness better than others. Nevertheless, if you are among those who become nasty, cynical, or even full of doubt when you are missing your sleep, you are morally obligated to try to get the sleep you need. We are whole, complicated beings; our physical existence is tied to our spiritual well-being, to our mental outlook, to our relationships with others, including our relationship with God. Sometimes the godliest thing you can do in the universe is get a good night’s sleep—not pray all night, but sleep. I’m certainly not denying that there may be a place for praying all night; I’m merely insisting that in the normal course of things, spiritual discipline obligates you get the sleep your body need.
~ D.A. Carson (Scandalous, 147)