By Ben Tinker, CNN

(CNN) -- Energy drinks are a $2.8 billion-a-year business in the United States alone, built on the promise of helping you push past that "2:30 feeling."

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.

Jamie Byrne, the study's lead author, says conventional wisdom has been that reward response is driven by "reward-related factors," such as the relative appeal of a reward ($10 vs. $100, for example) and "internal factors," such as whether you are an optimistic or pessimistic person.

"This study is testing a third component that may be relevant to this relationship: time of day," said Byrne, a Ph.D. candidate in clinical psychology at Swinburne. "Our best bet is that the brain is 'expecting' rewards at some times of day more than others, because it is adaptively primed by the circadian system."

Expected vs. unexpected rewards

Byrne and her colleagues recruited 16 healthy young men who worked normal daytime hours and hadn't done any recent long-haul travel, which could have resulted in jet lag. The men were asked to perform a gambling exercise while inside an MRI scanner, so blood flow in their brains could be monitored in real time.

The researchers said existing literature has found that an area of the brain known as the left putamen is a "core component of reward-related function in humans," and so they structured their study to observe subjects' activation in that area at three times of day: 10 a.m., 2 p.m. and 7 p.m. Of these, the lowest levels observed were at 2 p.m.

To put it simply: Rewards we receive in the morning or evening seem to come as more of a surprise than rewards we get in the afternoon. That surprise factor causes certain parts of the brain to light up more. This is a consequence of what is often referred to as our "primitive brain," from when our ancestors were hunter-gatherers. If they were going to venture out in search of food, for example, they would have done so during daylight hours; doing so at night would have presented an unnecessarily elevated risk.

"The human reward system is primed to be more active during daytime hours when reward potential is high and risk relatively low, and less active overnight when this balance is reversed," the researchers wrote, citing "Mood and Temperament," a 2000 book by University of Notre Dame psychology professor David Watson.

"Outside of 'time of day' variables, multiple lines of evidence suggest that neural activation is higher in rewards regions in response to unexpected rewards, compared to expected rewards," Byrne said. "A good analogy for this is your response to a surprise birthday party in comparison to a planned birthday dinner. Both are rewarding events; however, when the rewards are unexpected, your brain has to work harder to understand what is happening."

Implications for daily life

Prior research has demonstrated that it might be harder for you to think clearly, exercise good judgment and avoid making mistakes in the afternoon. This study is provides more evidence that it's not only what you do that matters, but when you do it.

Science has shown there are many ways to optimize daily life activities around your circadian rhythm.

Psychologist Eric Barker says you should always organize your to-do list from worst to best, knocking out the things you don't want to first, since "your self-control is at its peak first thing in the morning." Save the mindless tasks for the afternoon, he says.

"Some fitness gurus recommend working out first thing in the morning, because that's when you're least likely to have scheduling conflicts and therefore more likely to exercise regularly," says Robert J. Davis, author of "Fitter Faster." But you actually perform best at exercise later in the day, he says.

Even weight loss can be tied to when, not just what, you eat. "Skipping meals or eating too few calories earlier in the day appears to stack the odds against us," says nutritionist Lisa Drayer, a CNN contributor. "More and more research points to the fact that when you front-load your calories instead, you have a much better chance of shedding pounds."

Investigating how to better treat diseases affected by the body's internal clock -- such as depression, substance abuse and sleep disorders -- is something Byrne would like to study next.

"A range of evidence suggests that circadian rhythms are less robust in people vulnerable to depression and bipolar disorders, and we have shown that depression is indeed associated with a blunted circadian reward rhythm," she said.

Byrne thinks patients could benefit from a more precise focus on when they receive their treatment, maximizing rewarding experiences in the middle of the day and minimizing them at night.