There's a biological reason for the 'afternoon slump' -- and it's not what you had for lunch

The “afternoon slump” happens to the best of us. After 12 p.m., we can feel groggy, tired, and unmotivated while sitting at our desks.

This slump has often been blamed on having too much to eat at lunchtime, but a small, new study suggests it may just be down to how the brain is wired.

According to the new research, published in the the Journal of Neuroscience, we mentally stop seeking out rewards at about midday.

Our brains can be rewarded by a number of different things: food, learning new things, accomplishing challenges, and talking to people. For some reason, our motivation to seek out these things may dwindle around lunchtime.

We learn more when we’re not expecting to.

Researchers from the Society of Neuroscience examined 16 men, asking them to perform a gambling task at 10 a.m., 2 p.m., and 7 p.m., while comparing the activation of the reward systems in their brains with functional magnetic resonance imaging (fMRI) scans.

Fmri scans Byrne et al., The Journal of NeuroscienceActivation of a reward-processing brain region peaks in the morning and evening and dips at 2 p.m.

An area of the brain called the left putamen — the part that plays a role in the processing and expectation of rewards — was most activated early and late in the day and lowest in the early afternoon.

One reason the researchers suggest for what’s happening is that rewards are not expected in the morning and the evening, so activation in the left putamen is greater. In other words, when you don’t expect a reward is coming, you expend more energy to get one. At about midday, we are anticipating rewards — like lunchtime — so there is a slog in activation. As a result, somewhat paradoxically, we end up learning more in the mornings and evenings, because we’re not expecting to.

That’s just one explanation, and as the study was very small there is still reserach to be done in the area. However, the team believe that understanding the timing of reward responses could have implications for the treatment of certain mental disorders — such as depression, substance abuse, and sleep disturbances — which see people’s symptoms fluctuate throughout the day.

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