While some people are early birds and others are night owls, one thing many people have in common is a feeling of sluggishness in the afternoon. This is in large part due to your circadian rhythm, a roughly 24-hour "master clock" that regulates hormones in your brain -- including, most prominently, the ones that make you feel tired or awake. But you don't just experience a physical lack of energy. Your brain's reward processing system also takes a hit, according to a study published this week in The Journal of Neuroscience by researchers at Australia's Swinburne University of Technology. In the simplest terms, this system is responsible for helping you weigh potential risks vs. rewards, and arrive at a decision as far as what -- or what not -- to do.