Playing the Game — page 18
Playing the Game rules and reference.
Rolling a 1 or 20. When you roll a 1 on the d20 for a Death Saving Throw, you suffer two failures. If you roll a 20 on the d20, you regain 1 Hit Point.
Damage at 0 Hit Points. If you take any damage while you have 0 Hit Points, you suffer a Death Saving Throw failure. If the damage is from a Critical Hit, you suffer two failures instead. If the damage equals or exceeds your Hit Point maximum, you die. Stabilizing a Character You can take the Help action to try to stabilize a creature with 0 Hit Points, which requires a successful DC 10 Wisdom (Medicine) check.
A Stable creature doesn't make Death Saving Throws even though it has 0 Hit Points, but it still has the Unconscious condition. If the creature takes damage, it stops being Stable and starts making Death Saving Throws again. A Stable creature that isn't healed regains 1 Hit Point after 1d4 hours. Temporary Hit Points Some spells and other effects confer Temporary Hit Points, which are a buffer against losing actual Hit Points, as explained below. Lose Temporary Hit Points First If you have Temporary Hit Points and take damage, those points are lost first, and any leftover damage carries over to your Hit Points.
For example, if you have 5 Temporary Hit Points and take 7 damage, you lose those points and then lose 2 Hit Points. Duration Temporary Hit Points last until they're depleted or you finish a Long Rest (see "Rules Glossary"). They Don't Stack Temporary Hit Points can't be added together. If you have Temporary Hit Points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. For example, if a spell grants you 12 Temporary Hit Points when you already have 10, you can have 12 or 10, not 22.
They're Not Hit Points or Healing Temporary Hit Points can't be added to your Hit Points, healing can't restore them, and receiving Temporary Hit Points doesn't count as healing. Because Temporary Hit Points aren't Hit Points, a creature can be at full Hit Points and receive Temporary Hit Points.
If you have 0 Hit Points, receiving Temporary Hit Points doesn't restore you to consciousness. Only true healing can save you.