We all know very well that doing intense and heavy workout has a catabolic effect on muscles tissues. That means, it induces micro-level wear and tear in muscles fibers. Lots of inflammatory cells is released in blood stream as a beginning of this healing process which induces cortisol levels and your body cortisol levels goes up. This suppresses your serotonin, epinephrine, insulin and dopamine levels of your body. The suppression of these four important hormones suppresses growth hormones of your body which is a key factor of healing, repair and most importantly, having a synergetic effect on muscular hypertrophy. That means, your brain always makes your body to put some extra muscles cells or fibers during healing and repair process as a reinstatement of future similar damage. So, when you have a sound sleep your cortisol levels go down raising levels of those four important hormones. When this happens your growth hormones levels also goes up which is a key factor of gaining muscles cells and muscles fibers. Which has been regarded as muscular hypertrophy.
In conclusion, as much as you sleep nicely and more you have an anaerobic effect in your body. Which facilitates muscular hypertrophy.