Healing is the simplest form of losing Damage Points of any origin. Mental Damage and Physical Damage heal over time, through rest and time. There are things to push the speed in which these healings happen, mostly of arcane origin.
In time, without any other factor involved, characters only heal Mental Damage. One Mental Damage is healed per hour out of combat.
The most efficient way (other than using Mending or Ur'Raqiel) of healing is Rest. Mental Damage is healed automatically WIS per Rest (along the point per hour heal) and Physical Damage is healed through a Rest check.
A Rest check is rolled by Constitute and only works at a Rest of 8 hours. The 8 hour Rest can also be taken in steps over night, for instance like in camping and guarding, where shifts are taken, but only if no combat or large actions were taken within that timeframe.