With intense activity some tissue injury is expected. Growth hormone is produced with intense activity. The growth hormone helps you sleep, and during deep sleep repair is enhanced. The recovery time is fast enough that you can have a normal schedule, meaning you don’t have to to multiple days without working.
Sometimes we slip up and hurt ourselves a little worse. A more severe injury necessitates a more involved repair process that must include some period of time where the tissue is so weak and vulnerable recovery is greater than one day. So what provokes the repair response? It has to be some combination of the chemicals released from the tissue as well as the nervous system that regulates the repair process.