Rest. Time for muscle growth.
Two things are required for muscle growth: the first is optimal load during your training sessions and the second is quite long intervals between them because the process of muscle growth and recovery is taking place not during workouts but after them, during rest periods. That’s why you ‘go on’ doing bodybuilding even when you are sleeping because your muscles continue to grow.
The recovery process also consists of two parts: the first is deliverance from fatigue and micro-lacerations that occur in muscles after each training session. During this stage your muscles try to regain the state of strength and integrity they had before the workut. Fatigue of particular muscles appears to be just a part of the organism fatigue in common. Hard but the most effective workout exercises such as Barbell Squats, Dead Lifts or Bench Presses lead to higher level of fatigue than isolation exercises such as Dumbbell Triceps Extensions or Cable Crossovers. After each training session it is most of all important for the organism to overcome this purely physical fatigue. Only after this your organism can switch to the second part of recovery process – muscle growth and strength increase (to the so-called ‘super-compensation’).
That’s why I advise that you never go to a gym if you feel that you hadn’t rested enough after a previous training session. Never aggravate your fatigue by new load. Give your muscle chance to rest and grow!