Sydney, Aug 23 (IANS): Do you find yourself grumpy and less alert in the afternoon hours as opposed to the morning hours? It is because your brain expects a reward, while activation is greater in the morning and evening because rewards are not expected at these times, according to a study.

The findings showed that activation of a reward-processing brain region peaks in the morning and evening and dips at 2 p.m.

Understanding how activity in the brain's reward system changes throughout the day could have implications for depression, substance abuse and sleep disturbances, said Greg Murray from the Swinburne University in Australia.

For the study, published in The Journal of Neuroscience, the team compared activation of the brain's reward system in young men during a gambling task at 10 a.m., 2 p.m. and 7 p.m.

They found that activation in the left putamen was lowest in early afternoon. This finding may parallel the drop in alertness people tend to feel in mid-afternoon, the researchers said.

"While the left putamen had greatest activation at 10 a.m. and 7 p.m., it had significantly lowered activation at 2 p.m.," Jamie Byrne from the varsity was quoted as saying to Daily Mail.

"The left putamen is particularly responsive to unexpected rewards, so our data suggest that the brain's reward centres might be primed to expect rewards in the early afternoon and be surprised when they appear at the start and end of the day," Byrne added.

The results contrast with a previous research showing that people report being in the best mood -- a subjective measure of reward activation -- at this time, the researchers said.