Of all the methods of recovery to help mitigate cumulative fatigue, sleep trumps them all. Lack of sleep will inevitably lead to a decrease in performance and an increase in cortisol and a decrease in testosterone. Sub-optimal sleep also leads to fat gain and weight loss (in the form of muscle)! Most athletes generally need 8 hours of quality sleep/night; however, everyone is different, and some people do just fine with 6, others need 10. You should sleep the appropriate amount to where you feel rested and able to train again; you will not always feel pumped to go to the gym.