Studies have found that runners and swimmers who cut back their training by 60 percent showed no loss of endurance even after two to three weeks. Another study showed that strength losses weren't noticeable even a month after a three-week training program stopped.

Maybe you're the kind of guy who likes working yourself hard every day. What harm is there in continually pushing your limits?

It depends on how fond you are of feeling chronically tired, having trouble sleeping, being sick all the time with colds, flus and infections or feeling depressed. These are some of the signs of overtraining, a clinical condition in which you've pounded yourself so hard for so long that you've gone beyond your body's ability to quickly recuperate, says Dr. Wilson. Overtraining can also produce potentially more serious symptoms as well, such as high blood pressure and increased heart rate.

It's no coincidence that these same symptoms are tied to emotional stress. By overtraining, you physically stress yourself beyond your ability to cope and you burn out, just as you do after weeks or months of unrelieved out-of-control pressure at work. This relationship—in case you're thinking only elite athletes get overtrained—may make overtraining more common in nonelite, dedicated recreational athletes, according to doctors who treat the condition. That's because, unlike top competitors, with their sponsors and patrons, average Joes have to mix their training with the additional stresses of workaday life.

Treating overtraining is difficult. For one thing, its symptoms overlap with so many other conditions (Lyme disease is one), that it's often hard—but not impossible—to spot (see "Pushing or Punishing?"). Once it's identified, the main remedy is rest or reduced activity, says Dr. Wilson. In many cases, if the overtraining was minimal or caught early, a few days' recovery may be all that's needed, he says. If you're suffering from bona fide overtraining syndrome, however, recovery can take up to 12 weeks.

You've had a tough workout, you've set a personal distance record, you've hauled furniture all day: Better turn in early, right? Well . . . maybe. A review of different studies concluded that a single bout of exercise might, at most, make you sleep an extra ten minutes—barely a blip on the slumber charts.

Overall, being sleepy and being weary aren't the same thing, says Patrick J. O'Connor, Ph.D., who has studied the interaction of sleep and exercise as an associate professor in the Department of Exercise Science at the University of Georgia in Athens. Your body could be sore and aching, and you could feel physically exhausted, but that's a muscle matter, not a sleep issue. Muscles can rest even when you're awake. "If you're getting good sleep, exercise won't affect your normal needs or patterns much," Dr. O'Connor says.

But what constitutes "good" sleep? It varies from person to person, but most of us feel refreshed after seven or eight hours—assuming we get those seven or eight hours when it makes sense to our bodies. Sleep is governed by a number of daily rhythms, such as internal temperature, the release of hormones and the perception of daylight. Everybody's rhythms are a little different, says Dr. O'Connor. While it's broadly true that we sleep at night and are awake during the day, if you go to bed at 10 p.m. when you're accustomed to turning in at midnight, your sleep may suffer. But don't worry that you could be sleeping at a more optimal time than you do. "What we're accustomed to and what our bodies want are usually close to the same," Dr. O'Connor says.