If a hard day at work leaves you feeling unable to exercise, you can at least rest easy knowing there's a scientific explanation.

Using your willpower for one task depletes willpower for entirely different task, a new study finds. But there are strategies for getting it back, researchers say.

"Cognitive tasks, as well as emotional tasks such as regulating your emotions, can deplete your self-regulatory capacity to exercise," said study leader Kathleen Martin Ginis, associate professor of kinesiology at McMaster University.

The study used a Stroop test to deplete the self-regulatory capacity of volunteers in the study. The test consists of words associated with colors but printed in a different color. For example, the word "red" is printed in blue ink.) Subjects were asked to say the color on the screen, trying to resist the temptation to blurt out the printed word instead of the color itself.

"After we used this cognitive task to deplete participants' self-regulatory capacity, they didn't exercise as hard as participants who had not performed the task," Martin Ginis said.

The more people "dogged it" after the cognitive task, the more likely they were to skip their exercise sessions over the next 8 weeks. "You only have so much willpower," she said.

A similar conclusion was reached by a 2003 study in the journal Alcoholism: Clinical and Experimental Research. After completing tasks that required self-control, study subjects had less physical stamina and impulse control and increased difficulty with problem-solving activities. In particular, resisting temptation consumed an important resource that was then less available to help the person persist in the face of failure on subsequent tests.

But you can work on your willpower. "Willpower is like a muscle: it needs to be challenged to build itself," she said.

"There are strategies to help people rejuvenate after their self-regulation is depleted," she said. "Listening to music can help; and we also found that if you make specific plans to exercise — in other words, making a commitment to go for a walk at 7 p.m. every evening — then that had a high rate of success."

By constantly challenging yourself to resist a piece of chocolate cake, or to force yourself to study an extra half-hour each night, then you can actually increase your self-regulatory capacity, the researcher said.

Sleep may also be important. "Most forms of self-regulation failure escalate over the course of the day, becoming more likely and more frequent the longer the person has been deprived of sleep," said Florida State University researcher Roy F. Baumeister, who led the 2003 study.

The new study, detailed Thursday in the journal Psychology and Health, was funded by the Social Sciences and Humanities Research Council of Canada.