http://www.slate.com/articles/health_and_science/science/2015/09/caffeine_shifts_circadian_rhythms_in_the_brain_and_body_coffee_before_bedtime.html

Caffeine is America’s favorite (legal) white powder. You may know intimately the contours of its high: the jump in your step, the buzz in your brain, that jolt that renders you oh so gloriously and involuntarily alert. In a new study, researchers have quantified the effect of this potent stimulant on your circadian rhythm and explained how it works not just on your brain but also on the cells in your body.