Planescape - Fool's Road
Slow Natural Healing
Characters don’t regain hit points at the end of a long rest. Instead, a character can spend Hit Dice to heal at the end of a long rest, just as with a short rest.
This optional rule prolongs the amount of time that characters need to recover from their wounds without the benefits of magical healing and works well for grittier, more realistic campaigns.
Pg. 276 DMG