Do you get home at the end of the day and find yourself plunking down on the couch when you had every intention of heading to the gym or going for a run?

Your lack of willpower for a physical workout may be a direct result of the mental workout your brain got earlier in the day. A new study reports that stressful cognitive (thinking) tasks can weaken your resolve to exercise.

And some people may be more susceptible to this brain drain on self-control than others.

The study’s details
The study, which was published last Friday in the journal Psychology & Health (but won't be available to read online until Friday of this week), recruited 61 student volunteers, aged 18 to 30. All were non-exercisers. First, the students were asked to ride a stationary bike while the researchers measured their energy output.

Half the students were then given the Stroop test, which involves reading out loud a list of words for various colors that are printed in the “wrong” colors (the word red printed in blue, for example). This is a cognitively stressful test, requiring mental self-control. (Don’t believe it? Take a version of the test here.)

All the students were then asked to exercise again. Those who had taken the Stroop test exerted less energy during this second bout of cycling than the control group.

The researchers also asked the students to keep track of their planned exercise efforts over the next eight weeks.

“We found that the people who were most affected by having their self-control depleted in our experiment were also the most likely to skip exercising,” said Kathleen Martin Ginis, professor of kinesiology at McMaster University in Ontario, Canada, and the study’s lead author, in a phone interview on Monday. “They probably have the fewest strategies for replenishing their self-control both in our lab and in the real world.”

Strengthening your willpower
If you find yourself constantly derailing your best-laid schemes for exercising in the evening, don’t give up.

“Self-control is like a muscle,” said Martin Ginis. “The more you work it out — the more you exercise it — the stronger it becomes, and the bigger the pool of self-control you’ll have to draw on.”

Want to stiffen your resolve to hit the rubber on the treadmill rather than the “on” button on the TV remote? Here are a few willpower-building actions (and the reasoning behind them) recommended by Martin Ginis:

• Do something to put yourself in a good mood — listen to music that lifts your spirits or watch a funny YouTube video. “People who are in a good mood are better able to replenish their self-control,” Martin Ginis said.

• Have a quick, small snack. Your brain runs on glucose. Eating something can restock your brain’s store of glucose — and thus replenish its resources for exercising self-control.

• Get a good night’s sleep. Or, if you had a restless night, try taking a 10- to 15-minute power nap. Sleep is restorative for the brain.

• Plan your exercise in advance. Know when you’re going to exercise, where and for how long. Have your exercise clothes and equipment ready to put on and use. “Then, when it’s the end of the day and you’re debating whether or not to go exercise, you’ve already done the planning and have the self-control,” said Martin Ginis.

Of course, there’s always the option of exercising first thing in the morning, before your brain takes on any willpower-draining cognitive challenges. But, alas, for that option to work, as Martin Ginis pointed out, “you have to have the self-control to get out of bed in the morning.”

Oh, well.