If you're an overeater, for example, you're going to be storing more fuel than you need in the form of fat. And lugging around that excess body weight can make anyone feel sluggish. At the other extreme, undereating can also cause fatigue by depriving you of sufficient calories to propel your body through the day. They're like cars running on empty. Your activity level also has a direct effect on whether you feel fatigued, says LaForge.
Lack of exercise can easily create a pattern of inactivity that is difficult to break.
Letter carriers, for example, are always on their feet. Yet they complain of fatigue much less than office workers. On the other hand, too much exercise can have a negative effect. That's because when we exercise, the body produces lactic acid, a substance that accumulates in our muscles, producing weakness and body aches. This accumulation usually doesn't pose a problem when we avoid working ourselves to exhaustion and we follow our workouts with proper rest, because then our bodies are able to get rid of the lactic acid.