Lack of sleep leaves you feeling fatigued, achy and disoriented.  Scientists know that sleep is important but can't explain exactly why.  Brazillian scientists speculated that sleep is critical for protein synthesis.  Lack of sleep causes tissue breakdown marked by increases in catabolic hormones such as cortisol and decreases in anabolic hormones such as testosterone and insulin-like growth factor 1.  Inadequate sleep slows pathways involved in protein synthesis, increases protein breakdown, and promotes loss of muscle mass and muscle degradation.  In athletes, inadequate sleep slows injury repair and tissue recovery.  In older adults, sleep deprivation can contribute to the loss of muscle tissue.  (Medical Hypothesis, 77: 220-22, 2011)