If an elf is stable with 0 HP, do they regain all hit points after a long rest of 8 hours or 4?

Question

In my most recent session, the entire party except one character was downed (reduced to 0 HP), but they were all stabilised by the cleric (the only member that was left standing). One of the downed characters is an elf.

Because of the elf's Trance trait, normally when they sleep/meditate, it only takes 4 hours to finish a long rest. However, in this instance, they are not meditating, but just stable with 0 hp.

Would it take the elf 4 hours or 8 hours to finish a long rest and regain all their HP?

Answer 1

A stabilized elf can finish a long rest in 1d4+4 hours.

The rules for “Stabilizing a Creature” state (emphasis mine):

A stable creature doesn't make death saving throws, even though it has 0 hit points, but it does remain unconscious. The creature stops being stable, and must start making death saving throws again, if it takes any damage. A stable creature that isn't healed regains 1 hit point after 1d4 hours.

Only after the elf regains 1 hit point can the elf begin their 4-hour long rest:

a character must have at least 1 hit point at the start of the [long] rest to gain its benefits.

So an elf with 0 hit points can complete a long rest in 1d4+4 hours from the moment they are stabilized: 1d4 hours to regain 1 hit point¹, and 4 hours to complete the long rest.

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^{_{1 A non-negligible level of support has been given to the ruling that a creature can take a short rest while unconscious and spend hit dice to regain hit points after only one hour.}}

Answer 2

8 Hours.

The elf trance is often misunderstood as being that the long rest is only 4 hours whereas everyone else is 8 hours. The only thing the elf trance does is you need to sleep "trance" for 4 hours, but still need the remaining 4 hours to gain the benefit of a long rest such as other light activity.

The benefit is such that the elves can keep watch for longer (up to 4 hours). Whereas most other humanoids need 6 hours of sleep and 2 hours of light activity (or 8 hours of sleep if you like sleep ofc).

TLDR: 8 Hours - 4 hour trance + 4 hour light activity/downtime. Just for reference that elves do in fact need 8 hours for a long rest: https://www.sageadvice.eu/elf-trance-does-it-affect-long-rest-for-elves/

EDIT

Thanks to Sarge in the comments, it seems Eladrin, Shadar-Kai and Sea Elves gain the benefit of a long rest after 4 hours, either by an error in MPMM or a change in the desired D&D design. I found no errata that changes the phrasing of the Trance for Eladrin & co, nor errata that changes the phrasing of the original PHB.

Conclusion

Long rest: A character can’t benefit from more than one Long Rest in a 24-hour period, and a character must have at least 1 hit point at the start of the rest to gain its benefits.
The elf will be at 0 hit points until healed or unconcious for 1d4 hours, then 8 hour long rest if an elf from PHB or 4 hour long rest if Eladrin, Sea elf or Shadar-Kai.