Mid-afternoon slump affects brain’s reward system
New research may affect our understanding of the effect of circadian rhythms on mood, mental health and other brain activity. Angus Bezzina reports.
It’s not just wakefulness that hits a slump in the mid afternoon, according to new research: also experiencing a dip in activity around 2pm is an area of the brain that is essential to reward-processing.
This is a noteworthy development because it contrasts with previous research in which people have reported experiencing their best mood - a subjective measure of reward-processing activity - at this time.
To come to this conclusion, PhD student Jamie Byrne and her supervisor Greg Murray, from Swinburne University in Melbourne, and colleagues analysed and compared the activity of the reward-related brain regions of 16 healthy men while they were participating in a gambling task on 3 occasions over the course of a day - at 10am, 2pm and 7pm.
Using a neuroimaging technique known as blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI), the researchers found that the activity within the left putamen - a large structure at the base of the forebrain - was lowest in the early afternoon.
In their paper, published in The Journal of Neuroscience, the researchers claim that the reason activation is lower in the afternoon than in the morning or evening is because the body expects rewards to be more abundant at that time and reduces the amount of neural excitation as a result.
The paper argues that this reward expectation is driven by the circadian system - the network that regulates body rhythm, normal sleep and wake phases - but note that the precise neural mechanisms of this relationship are still not well understood.
This research has the potential to significantly impact the treatment of conditions like depression, insomnia and substance abuse for which symptoms can vary across the day. It will also challenge neuroimaging studies that failed to control for or repeat the time of day when scanning took place.