“People tend to think muscles build while you workout. This is not true, in fact they repair and build up when you rest and are not putting them at work. Similar to how you wake up in the morning with that fuzzy stuff in your teeth, eye dirt and foul breath. While you sleep, your body “cleans” and “repairs” itself of waste when it is not working and spending it’s time expelling energy. Your muscles work the same way. After a workout, muscles will want to start reparations as soon as possible. This is why it’s important to allow yourself time to rest and focus on aftercare.”
Source: wikihow.com