Most of the time the source of the fatigue is temporary—late nights at work; a new baby in the house; exam time; a stressful relationship. Regardless of the cause, if you don't get on top of it, and set in place some steps to counteract the fatigue, you'll may go into that non-functioning zombie state from pure exhaustion.
Runner's can experience fatigue as well. It can be the stress-of-life-induced fatigue, but there can also be many other more running-specific sources for the fatigue.
One of the main causes of fatigue in runners is overtraining. Overtraining happens when you don't provide your body ample amount of recovery time. This often happens with runners using and over zealous marathon (or any race distance) training program. A good rule of thumb to follow when training is to alternate hard and easy days. Hard and Easy don't always refer to distance. The terms also refer to intensity. A short run can be "easy" if it's a 3-4 mile run at a conversational pace. Or short can mean "hard" if it's an interval workout at the track. Long can be "easy" if it's a leisurely 10-miler or it can be "hard" if it's a 10-miler at race pace.
Throwing in a day or two of cross-training is a great way to give yourself an aerobic workout while giving your "running muscles" a break. And even a complete day of rest (meaning not doing anything) once or twice a week is much more beneficial that going full steam 7-days a week.
Overtraining can also be due to repetitive exercise when resistance training. If you don't vary your workouts and you're constantly subjecting your body to the same stress over and over, those muscles can become overtrained. A good rule of thumb is to wait at least 48 hours before working the same muscle groups again. So for example, if your do a chest/triceps workout one day, you should wait at least 2 days before working those muscles again. Professional bodybuilders will often workout a muscle group so hard in one workout, that they'll wait an entire week before working that muscle group again.
An elevated pulse is a good indicator of possible overtraining or even sickness such as a respiratory infection. If your waking resting pulse is elevated more than a few beats, you could have an infection or be suffering from overtraining. In either case, taking a day off may be the best thing. Rest is the best thing for overcoming overtraining. If rest doesn't do the trick, schedule an appointment with your doctor.
Improper hydration can also be a source of fatigue. Most people in general don't get enough water (2-3 liters) each day. If you fall into that categore and you're also not replacing the water you're losing through perspiration from running, you're risking dehydrated. A sure sign of dehydration is fatigue. In addition to your normal daily hydration requirements, you should drink 12-16oz of water about an hour before your run. One good way to determine how much you need to drink after your run is to weigh yourself before your run, then weigh yourself immediately after your run. For every pound lost, you should drink 16oz of water or sports drink. Of course you can't do this after every run, but if you do it on a mild day and once on a really hot/humid day, then you'll have a frame of reference to help you determine how much to drink after runs in various conditions.
If you're running less than an hour, water is perfect. If you're running an hour or longer, a sports drink will be a better choice especially on hot/humid days because it will help replace vital electrolytes lost through sweating.
Lack of sleep, is a big-time cause of fatigue. Your body does most of it's repair and rebuilding while you sleep. If you're not getting enough sleep, then you're not giving your body time to heal. Plain and simple. Sleep requirements can vary from person to person. Teenagers need about 9 hours on average (mine seem to need about 15!). Most adults need 7 to 8 hours a night for the best amount of sleep, although some people may need as few as 5 hours or as many as 10 hours of sleep each day. Fatigue can result when your normal sleeping hours are shortend for whatever reason—stress of a new job, a new baby, or that heartburn you got from the 5-meat pizza you ate just before bed. If you're not getting your normal amount of sleep, then you need to back off on your training until your sleep hours are back to normal.
Low iron levels can be another cause of fatigue. If you've ruled out other possible causes of fatigue, it may be worth having your doc take a blood test to check your iron levels. This can especially be problematic for some women during their menstrual cycle. Sometimes just a change in diet can help boost your iron levels, but sometimes an iron supplement may be needed. (Check with your doctor before taking an iron supplement.) Good food sources of iron include: turkey, clams, enriched breakfast cereals, beans/lentils, pumpkin seeds, blackstrap molasses, canned beans, baked potato with skin, enriched pasta, canned asparagus
Sometimes you may not experience the fatigue during your run. For some the fatigue may come after the run. Insufficient post-run refueling can be the culprit. If you've had an intense workout, it's normal to feel tired, but if you're feeling fatigue that just won't go away, you may not be giving your body enough refueling carbs and protein after your run. A good rule of thumb is to consume a 4:1 ratio of carbs and protein within 30-45 minutes of finishing your workout. Oddly enough, lowfat chocolate milk has the optimal ratio of carbs to protein to help refuel tired muscles.
So, if you're experiencing fatigue, whip out this list and see if you can narrow it down. If this list doesn't pin down the cause for you, make an appointment with your doc.