Exercise is not simply sweating off calories. A good training program applies stress to the body, and the body responds with a desired adaptation (improved strength, power, or metabolic conditioning). However, there's a catch. Our bodies do not have an infinite capacity for stress. When this capacity is exceeded the ability to develop adaptations (also known as gainz) is negated.
We can think of the body's capacity as a bucket, and stress as water in the bucket. Now, here's the thing. Exercise isn't the only stressor filling up the bucket. Stress can come from...
If our stress bucket is already filled to the rim from these sources, the intentional stress from exercise will spill over onto the floor... wasted. No positive adaptations will occur, and we may even become ill. If you consistently don't feel energized going into your workouts your stress levels may be maxed out. If that's the case, do a personal inventory of the items I listed above to identify areas where you can make incremental improvements. Every little drop helps.