The next time you're yawning at your desk around midafternoon, take comfort in the fact that the reward center of your brain is yawning, too.

A small study out of Australia's Swinburne University of Technology found that the region of our brain associated with rewards — the left putamen — slumps in the afternoon, especially when compared to the left putamen's activity in the morning or in the evening.

To come to this conclusion, researchers had 16 healthy men who hadn't engaged in any long-haul travel — wouldn't want jet lag messing with the results — engage in a card-guessing activity at 10 a.m., 2 p.m. and 7 p.m. The participants received a financial bonus for their best guesses in each round.

The men were hooked up to a functional Magnetic Resonance Imaging (fMRI) machine during the test so their brain activity could be monitored.

"We found that activations in the left putamen, the reward centre located at the base of the forebrain, were consistently lowest at the 2 p.m. measurement compared to the start and end of the day," Jamie Byrne, Ph.D. candidate and the study's lead author, said in a statement.

"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 body clock."

Basically, the brain's reward expectations are governed by the body's circadian rhythms, the same way wakefulness hormones are. It doesn't expect a reward of some kind in the morning or the evening, because of the time of day it is, but it does expect it during the day.

Byrne likened the brain's responses to how you'd respond to two different sorts of birthday celebrations. A surprise party means your brain is working a bit more to contextualize the event — it surprised you, after all — while a planned birthday dinner is an expected activity. You're likely to enjoy both, but your brain has to do more to understand the surprise party.

So if someone surprises you at the office with cake at 2 p.m., you brain may just think, "Well, yeah, it's 2 p.m. That's cake time. Whatever. Try me at 10 a.m. I won't be expecting cake during bagel time."

Beyond being helpful when it comes to planning office events, Byrne and her colleagues think that their findings could have mental health implications, too. Given that the left putamen is more active at certain times of the day, treatments for depression and bipolar disorder may need to be adjusted to complement those active periods.

This is why you're so tired around 2 p.m.
A small study found that the brain's reward center dips in activity the same we typically want a nap in midafternoon.