Whenever I have an especially hard day at work, I know my workout is going to go one of two ways: Either I'm so pumped to finally get out of the office I sprint two miles on the treadmill or I'm so mentally exhausted I can barely step up on the machine. I always wonder why that's the case. After all, it's not like I do anything physically exhausting at my work. Outside of making tea (and taking subsequent bathroom trips), I rarely even get up from my desk. But it's all starting to make sense now thanks to new research from Bangor University.

In the study, participants who performed a mentally-fatiguing task prior to working out reached exhaustion more quickly than those who were mentally rested. What's interesting is that researchers found that mental fatigue had no affect on the performance of the muscles or heart. So basically, that mental exhaustion you think is hampering your workout really is all in your head.

"The participants in the mentally fatigued trial perceived [that their] exertion was higher even though physiologically their effort was the same [as the non-fatigued trial]," says Jessica Matthews, continuing education coordinator and certified personal trainer for the American Council on Exercise.

So if you've had a particularly taxing day at work, will it really affect your workout performance?

"Individuals who engage in high intensity training, such as a competitive athlete, may want to complete their training while they're mentally rested," says Matthews. As for the rest of us mere mortals? Turns out it really doesn't matter.

Because most of us aren't professional athletes and exercise at a moderate to high intensity, being mentally rested won't have a huge affect on calorie burn, says Matthews.

And what about morning workouts? Do they really burn more calories?

Nope.

While working out in the morning can help prevent exercise from sinking on your to-do list as the day progresses, you burn about the same amount of calories whether your workout is morning, noon or night.

"What it really boils down to is the number of calories in versus the number of calories out. This is what we refer to as negative energy balance," says Matthews. "Choose a time of day which fits your schedule and where you'll feel you'll be most successful. You want to enjoy the stress-relieving benefits of exercise, and if you feel rushed in the morning, having to workout is going to add an extra layer of stress."

The real key to calorie-burn, Matthews says, is less about when you work out and more about participating in an activity you like. So whether you like pacing yourself on the treadmill, gliding on the elliptical or kicking butt in kickboxing, find something you enjoy and make it fit into your life, not the other way around.