You do need to rest, especially just after each treatment cycle. But exercise improves the quality of your rest and your sleep. When you relax or sleep after exercise, your body rebuilds itself from the stimulation of deep breathing, cardiovascular exertion, and resistance training. Your body will get stronger, not weaker, and better able to recover. Exercise will help give you the physical and mental strength and endurance to counteract the weakening ordeal of chemotherapy and radiation.