Exercise is a great habit to have within your daily life; however, when it becomes an obsession it can actually become counter-productive to your overall health. Excessive training (in the form of volume and/or intensity) without adequate rest causes the body to become "numb" to external indicators of over training such as mood swings, simple sugar cravings, interrupted sleep, loss of sex drive, loss of body weight, suppressed appetite and an elevated resting heart rate.
Research indicates that after 12 weeks of consistent training, Cytochome C (a mitochondrial enzyme involved in the production of energy at a cellular level), reaches a peak and then beings to decline. In addition to Cytochrome C levels, so does your maximum oxygen uptake (also known as your VO2 Max.). At this point, the body must be allowed to rest and re-group for continued progress.
Give it a rest!
Training creates adaptations within the body's various systems (muscular, cardio-pulmonary, lymphatic, nervous and connective) and needs to be supported with rest and food for positive adaptations. Inadequate amounts (and quality) of sleep and food set the body up for a physical break down which leads to negative effects on the body (i.e. suppressed immune system and muscles with less power and endurance).
In addition to adaptations within the body's systems, training causes changes at a cellular level - cell mitochondria swell, metabolic wastes accumulate, essential nutrients (particularly electrolytes and stored glycogen) deplete, and muscle tissue is torn. This tearing is known as microtrauma of the cells, and torn muscle tissue doesn't work efficiently. As popularly noted, it takes 48 hours for the body to recover from this micro-trauma and has to be supported with rest and food for proper recovery and improved overall health.
If the body doesn't get the opportunity to rebuild from the "work phase" of training, overall health and associated performance begin to slow down (and in extreme circumstances, cease all together).
The concept of hard training days followed with easy-active recovery days incorporated into your weekly training schedule establishes the balance necessary for maximum improvements in your overall health and ultimately your performance. Consistent training without physical or mental setbacks provides the foundation for your body absorb your training volumes. The larger the foundation (i.e. quality of overall health) the quicker you will recover from workouts and the quicker your body will progress to new levels of performance.
The key to overcoming your fear of taking time off is to understand how much it will help, rather than hinder, your performance. Think about it this way, if you are not fresh, you will not have the energy (or desire) to push to the next level of performance. If your body doesn't experience the next level, you will begin to stagnate within your performance cycles. So, the next time you see a recovery workout on your schedule, don't ignore it! Remember, that rest allows your body to recover, rebuild, and ultimately become stronger.