For most people, the sleep-wake cycle creates an energy dip between two and four A.M. (when they’re asleep) and another one between about one and three P.M. (postlunch). If you’re sleep-deprived—which refers not just to the amount of sleep you’ve gotten over the past few days but also to the quality of that sleep—you will tend to notice the postlunch dip more. And you’ll notice it more as you age.