"Rest is a very important part of all exercise and athletic programs," says Tim Lawrence, a physical therapist in Hampton, Va., who has worked with professional athletes.
Build in recovery times: Aerobic activities such as running, swimming or step aerobics require shorter rest periods, usually less than 24 hours, but the body needs about two days to recover and repair muscles after weight training and other workouts that involve heavy resistance.
Rest a fever: Exercise further increases the body's internal temperature, which can make an illness worse. You also may need a day off if you have abdominal unrest -- vomiting and diarrhea can cause dehydration -- or a cough or congestion that makes breathing more difficult.
Don't push through too much pain: When a joint hurts to the point that easy movements are difficult, stop exercising. See a doctor if the pain or swelling doesn't improve with two or three days of rest.
But don't be too tentative: When muscles are sore after a workout, they actually can benefit from repeating a workout. But if the level of soreness doesn't lessen after a similar workout -- or one of lesser intensity -- rest as if you're injured.
Be smart about sleep: Some trainers recommend hitting the snooze button if you haven't slept well and wake up exhausted. If your usual routine is to work out early, however, you might feel better (and more energetic) if you get up anyway.
Watch for symptoms of heat illness: If you feel dizzy, nauseated or clammy, or develop a headache or persistent muscle cramps, stop exercising, drink water and find a cool area to rest. Be aware that heat exhaustion can strike even on mild days.