Realistic Healing (5e Variant Rule)
From D&D Wiki
In the standard rules of D&D, a creature will regain all of its hit points after a long rest as long as no external effect is preventing such a thing. This will occur even after said creature was on the brink of death. Even a creature that received near-lethal damage and did not receive any magical healing afterwards will make a full recovery within eight hours if it so chooses (twelve if the creature was at 0 hit points). Needless to say, this is not how the biology of any nonmagical living being works.
The Healing Process
With this new variant rule, a creature does not regain all of its lost hit points in a single long rest and cannot regain hit points and spent hit dice within the same long rest.
Instead, the creature must choose to either regain hit dice, or be healed. If it chooses hit dice, the creature regains half of its maximum hit dice rounded up as usual. If it chooses hit points, the creature rolls any number of hit dice they have available.
Hit points represent not only how good of condition a creature is in, but also how well any existing wounds are patched up and how well a creature recovers from the initial shock of the injury, and these are precisely what hit dice are for. However, there is only so far treating one’s wounds can go, which is why hit dice are limited in the manner that they are.
How Magic Affects Things
Supernatural abilities can greatly accelerate the healing process. If any magical or psionic healing is used on a creature, it regains additional hit points during its next long rest equal to the number of Hit dice used, plus its constitution modifier, plus the spell’s level or the number of psi points spent.
Returning from the Brink
Recovering from large amounts of damage can still be relatively quick compared to coming back all the way from death’s edge, which can take a bit more to recover from, although the adrenaline rush from being so close to death can delay the effects. When a creature is reduced to 0 hit points and is not stabilized or given magical healing by the end of its next turn, it suffers a level of exhaustion one minute later.