Although muscle stimulation occurs in the gym lifting weights, muscle growth occurs afterward during rest. Without adequate rest and sleep, muscles do not have an opportunity to recover and build. About eight hours of sleep a night is desirable for the bodybuilder to be refreshed, although this varies from person to person. Additionally, many athletes find a daytime nap further increases their body's ability to build muscle. Some bodybuilders take several naps per day, during peak anabolic phases and during catabolic phases.
Overtraining occurs when a bodybuilder has trained to the point where his workload exceeds his recovery capacity. There are many reasons that overtraining occurs, including lack of adequate nutrition, lack of recovery time between workouts, insufficient sleep, and training at a high intensity for too long (a lack of splitting apart workouts). Training at a high intensity too frequently also stimulates the central nervous system and can result in a hyper-adrenergic state that interferes with sleep patterns. To avoid overtraining, intense frequent training must be met with at least an equal amount of purposeful recovery. Timely provision of carbohydrates, proteins, and various micronutrients such as vitamins, minerals, phytochemicals, even nutritional supplements are acutely critical.
It has been argued that overtraining can be beneficial. One article published by Muscle & Fitness magazine stated that you can "Overtrain for Big Gains". It suggested that if one is planning a restful holiday and they do not wish to inhibit their bodybuilding lifestyle too much, they should overtrain before taking the holiday, so the body can rest easily and recuperate and grow. Overtraining can be used advantageously, as when a bodybuilder is purposely overtrained for a brief period of time to super compensate during a regeneration phase. These are known as "shock micro-cycles" and were a key training technique used by Soviet athletes. However, the vast majority of overtraining that occurs in average bodybuilders is generally unplanned and completely unnecessary.