The dogs that were left to rest and not ordered to stay spent on average almost three times longer on the next activity. Their mental energy had not been worn down - they hadn't had to obey the command beforehand.
The short of it is: we can't keep expecting to make good choices and exert self-control without running out of energy. This is what experts call "decision fatigue", which can hit whenever we have a string of choices to make. Unlike being physically tired, we may not notice as it sneaks up on us.
Picture yourself doing the weekly shop. You cruise the aisles, making choice after choice along the way. Many are habitual and some are mental shortcuts, like reaching for familiar brands or sticking to a list. But because prices change and we typically want variety in what we eat, there are always active decisions being made.
And then we get to the check-out, and wouldn't you know it, there are all those snacks on display! Another choice to make - to buy or not to buy?
We're more likely to act impulsively when mentally tired.
All those decisions sap our willpower - much like the dog who was ordered to stay those 10 long minutes. Shop owners figured out long ago that we'll buy at that point, and now research explains why.
It turns out that glucose - and this has been proven in people as well - is the fuel for willpower and self-control.
Speaking of sugary snacks, the dog study went much further to show something remarkable. Repeating the same experiment with more pairs of dogs, they found that the obeying dogs' willpower was restored when they gave them a boost of glucose. A sugar hit was all it took to erase the differences previously noted in the dogs.
It turns out that glucose - and this has been proven in people as well - is the fuel for willpower and self-control. And particularly when our brains get motoring with challenging choices to make, we need to keep filling the tank.
Perhaps we should leave all our most important money decisions until right after breakfast. After all, we make better, more informed choices when we're not tired. Or set goals with our partner over dessert! (But perhaps not over a drink, since alcohol reduces the glucose in our system.)
Think back to the grocery checkout, after all those choices in the aisles.
As dieters will experience as well, we ironically find ourselves between a rock - needing the willpower to avoid the sugar - and a hard place - needing the sugar to have the willpower!
Meanwhile the dog rests outside.
Get Sorted is written by Sorted's resident blogger, Tom Hartmann. Check out the guides and calculators at Sorted - brought to you by the Commission for Financial Capability - at sorted.org.nz. 
