It's not just you. Exercising after a stressful day really does feel more difficult — with the key word being "feel." New research found that being mentally fatigued makes a cardio workout feel harder. After engaging in a difficult computer task for 90 minutes, study participants reached the point of exhaustion earlier than when they watched a dull documentary prior to exercising. Interestingly enough, the participants' actual cardio capacity was not affected. Seems like a case of mind over body, with the mind winning even though the body is still quite capable. Moral of this tale is: Don't avoid the gym after a hard day's work, your body really needs to move after experiencing all that stress. But do know that you might have to psych yourself up more than usual. Try taking a group fitness class and be buoyed by the motivation of the group.