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Brain reward region slumps in afternoon – and it could affect mental health

A new study shows that activity in a reward region of the brain slumps in the early afternoon – and it could offer clues to why people feel tired after lunch.

Activity in the reward-processing area peaks in the morning and evening, but drops off at around 2 p.m., researchers from the Swinburne University of Technology in Australia discovered.

The neuroscientists, led by Dr. Greg Murray, studied the brain activity of 16 healthy young males as they took part in a gambling task at 10 a.m., 2 p.m. and 7 p.m. What they found: activation of the left putamen, a structure located at the base of the forebrain, was lowest during the 2 p.m. task.

The findings contrast with previous studies that show people report being in the best mood in the early afternoon. The researchers on the new study suspect that activation might be higher in the morning and evening because there is no expectation of a reward during that time.

The timing could mean clues toward treating depression, sleep disorders, addiction and other mood disorders, the scientists said. Patients with these disorders often report symptoms that fluctuate throughout the day. The putamen plays a role in the reward region of the brain that earlier research has connected to addiction and depression.

The new study also shows that doctors may want to consider when they conduct imaging studies of their patients’ brains. Scans taken during the morning may look different from scans taken during the afternoon in the same patient.

The results may also offer some insight into why some people have an “afternoon slump” after lunch. The sleepy feeling some people report around 2 or 3 p.m. is part of the body’s natural rhythms, doctors believe.

The study will be published in the Journal of Neuroscience. Funding came from the Barbara Dicker Brain Sciences Foundation.

By Kyla Cathey, staff writer

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