Researchers say the reward system in our brains is least active during the mid-afternoon, leading to that tired feeling you experience at about 2 p.m.

Wonder why you always seem a little tired in the middle of the afternoon?

Don’t worry.

That “afternoon slump” might simply be part of your brain’s reward system.

And, as our understanding improves of how the brain responds to rewards differently throughout the day, it may also lead to new ways of dealing with mood disorders.

That understanding got a boost from a new study that concludes the brain’s reward system is least active around 2 p.m.

The new findings may eventually help improve the treatment of conditions with symptoms that vary throughout the day.

This includes disorders like depression and substance abuse.

The findings, published earlier this month in the Journal of Neuroscience, also build on a growing — but sometimes conflicting — understanding of how and why people feel and act during the post-lunch hours.

For the new study, a team of Australian researchers measured activity in a region of the brain thought to be related to reward responses in 16 young men.

The men engaged in a gambling task — an activity with a clear reward — at 10 a.m., 2 p.m., and 7 p.m.

Previous research has shown that the parts of the brain responsible for desire and positive emotions are more active during the day, when the potential for rewards is high, as opposed to during the evening, when rewards are less likely.

Reward function is also known to be subject to circadian rhythms in animals, said Jamie Byrne, a clinical psychology PhD candidate at Australia’s Swinburne University of Technology, and lead author of the new study.

“We wanted to know if we could measure circadian modulation of reward in the human brain and weren’t sure what to expect,” Byrne told Healthline.

They found that activation of the brain regions associated with reward was significantly greater at 10 a.m. and 7 p.m. than in the afternoon tests.

This, they conclude in their paper, showed “greater activation to reward stimuli at unexpected times of day.”

It’s a bit confusing, but because rewards are more likely, and thus more expected, during the afternoon, the brain’s reward system downshifts into a lower gear then.

In essence, the brain feels it doesn’t have to work as hard to find those rewards.

“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,” Byrne said.

That could explain why the findings appear to contradict those of previous studies that have relied on self-reported reward responses and found positive responses to rewards peaked in the early afternoon.

Byrne explained the difference with an analogy.

Both a surprise birthday party and a planned party are rewarding, but, she said the brain has to work harder and use more oxygen to understand what is happening in the surprise party.

The blood oxygen level-dependent fMRI that the researchers used to analyze reward responses in this study — to get objective responses rather than the subjective ones self-reported in previous studies — capture that oxygen use.

One of the main takeaways, the researchers say, is that studies that rely on neuroimaging to understand what the brain is doing during certain activities should take into account the time of day the activities are done.

Another is that greater understanding of how reward responses vary could lead to more precise treatments for conditions like bipolar disorder, depression, or substance abuse.

“For example, can we decrease vulnerability to relapse in people with bipolar disorder by controlling the daily timing of rewards (by) maximizing rewarding experiences in the middle of the day, and minimizing rewards in the evening hours?” Byrne said.

Further knowledge about reward responses and how they vary over the course of the day could help patients control their own desires or impulses.

“Patients might want to understand that rewards may be more enticing at certain times of day than others,” said University of California Berkeley psychology professor Sheri Johnson, PhD, a co-author on the new study. “That might help them think about peak periods when they would be at risk for impulsive responses to rewards.”

But for now, Johnson said, “we seem far away from knowing enough to plan interventions.”