Even if you're feeling more energetic by day three or four, go easier on exercise than you normally do, not harder. You don't want to tap out your energy stores or you'll risk sending your body into starvation mode. Haas says it's okay to do some light cardio while cleansing. "My aerobic capacity is actually better when I'm on my yearly juice cleanse," he says.
But skip the weight room. There's a good chance you're muscle strength won't be all there, and you don't want to overtax your body. If you still have energy to burn after a light workout, clean the kitchen or straighten that mess in the garage you've been avoiding.