Don't blame that long lunch meeting or low caffeine levels for your lack of care-factor in the early afternoon.
New research suggests your brain's reward functions are tuned in to your body's daily rhythms, with 2 pm being the low-point in your ability to feel good about nailing that big, important task.
The post-lunch slump is a familiar feeling for many people, often resulting in either a siesta or a masculine-sounding fizzy beverage.
Researchers from Swinburne University of Technology in Australia decided to look into just how much of our desire to commit to a task was linked to our body's natural circadian rhythm.
Using an imaging technique called blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI), the team investigated how parts of the brain responded to a sense of reward at different times of the day in a group of 16 healthy, right-handed men.
The volunteers were given a task where they had to guess whether the hidden value of a digital card on a screen was higher or lower than five. Guessing correctly earned a reward, while an incorrect guess was penalised.
To motivate the volunteers, the researchers told them they would receive a bonus for the best of three trials conducted at three different times; 10 am, 2 pm, and 7 pm.
Watching their brains as they performed the task revealed a pattern of activity in a structure at the base of the brain called the putamen.
This round blob of brain tissue manages a number of tasks, many revolving around learning and reinforcing behaviours that give the best chance of a good outcome. Having it primed at certain times over others can be interpreted as being ready to deal with an unexpected reward.
"We found that activations in the left putamen, the reward centre located at the base of the forebrain, were consistently lowest at the 2 pm measurement compared to the start and end of the day," says researcher Jamie Byrne.“
This is in line with their hypothesis, which claimed the brain's response to rewards varies according to the same biological rhythm that encourages us to wake, sleep, or feel hungry during the day.
In other words, our brain isn't expecting a task to be as rewarding in the morning and late in the evening as it is in the middle of the day.
The research doesn't provide evidence explaining the reasons why our brains are primed to learn from experiences better at the beginning and end of a day. It was also only on a small group of males, so there's still plenty of room for debate.
It's possible that these are high-risk times, where our bodies are tired, hungry, or exposed to more threats.
Having a brain that's primed to deal with problems that could result in unexpected rewards at certain times of day could save time and maximise benefits.
For the rest of us, the research is a convenient excuse to take an extra hour to socialise in the lunch-room.
Just tell the boss your brain is expecting a better reward.