How does a Flameskull's Rejuvenation affect its rest?

Question

Flameskulls are interesting creatures in that they combine three traits - they are undead, they have the Spellcasting trait with wizard spells and spell slots, and they are not killed at 0 HP. Rather, they are "destroyed" but Rejuvenate after one hour.

A previous question has asked whether the 'rejuvination' restores their spent spell slots and concluded it does not, by the principle of abilities do only what they say they do.

If they must recover their spell slots by rest, do Flameskulls need to sleep to do so? A previous question about undead in general suggested that while their Undead Nature means they are not required to sleep, that does not mean that they cannot sleep. Thus as NPC casters recovering spell slots, (and without access to features like Arcane Recovery or the Lair Action of a Lich that recovers spell slots), Flameskulls would need to take a long rest, which would include voluntarily sleeping.

Suppose a Flameskull is down a few slots and is engaged in rest and possibly sleep to recover them. It is attacked and 'destroyed', and then Rejuvenated in an hour.

Does the time that it spent while destroyed...

a) allow it to continue to both rest and sleep, so that it has gained an hour more of each when it is rejuvenated?

b) allow it to continue to rest but not to sleep, so that it has an hour more of rest but not of sleep when it is rejuvenated?

c) not permit it to either rest or sleep, but not remove any time for these it had already 'accumulated'? (It is thus 'paused' for an hour)

d) 'interrupt' both its rest and sleep, so that even if it had hours of these banked, it has to start over upon rejuvenation?

For reference, from the PHB errata:

A long rest is a period of extended downtime, at least 8 hours long, during which a [creature] sleeps for at least 6 hours and performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch.”

From the PHB

If the rest is interrupted by a period of strenuous activity—at least 1 hour of walking, fighting, casting spells, or similar adventuring activity— the [creatures] must begin the rest again to gain any benefit from it...a [creature] must have at least 1 hit point at the start of the rest to gain its benefits.

Note that 'being destroyed' is not explicitly one of the 'similar adventuring activities' that interrupts a rest. The requirement of having at least one hit point would rule out the Flameskull starting its long rest immediately upon being destroyed, but does not explicitly prevent one from continuing a long rest if it had already begun one when it was destroyed.

Note that, unlike rest, "sleep" is not a game-defined condition, but a state that is meant to be intuitively adjucated on the fly (DMG p. 248).

Answer 1

The time spent by the Flameskull "rejuvenating" is time that it is not a creature, and therefore cannot rest. The rest must begin anew once it has reformed.

In your question, there is the assertion

Flameskulls would need to take a long rest, which would include voluntarily sleeping.

To address this assertion we first we need to answer the subquestion:

Does a Flameskull need to meet the sleep requirement of a long rest?

This is a case of specific vs general.

The general rules for a long rest are (as you quoted):

A long rest is a period of extended downtime, at least 8 hours long, during which a [creature] sleeps for at least 6 hours and performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch.

Due to their Undead Nature feature:

Undead Nature. A flameskull doesn’t require air, food, drink, or sleep.

The flame skull doesn't need to meet the sleep requirement for a long rest (similar to the way elves trance feature obviates their need for sleep).

As a result there are two options:

1. The flame skull must rest for 8 hours (no sleep required) including no more than 2 hours of light activity, to regenerate their spell slots
2. The flameskull must rest for 2 hours during which they can only do light activities to regenerate their spell slots

As described in the accepted answer for the linked question, the elves trance feature means they need only to meditate for 4 hours to satisfy the conditions for a long rest.

According to the 2017 update to the Sage Advice Compendium:

Q: Does the Trance trait allow an elf to finish a long rest in 4 hours?

A: If an elf meditates during a long rest (as described in the Trance trait), the elf finishes the rest after only 4 hours. A meditating elf otherwise follows all the rules for a long rest; only the duration is changed. This answer has been altered as a result of a tweak to the rules for a long rest, which appears in newer printings of the Player’s Handbook.

This ruling reverses guidance in the earlier version of the SAC, due to errata changing the rules for long rests.

This ruling is based on the following line from the Trance feature:

After resting in this way, you gain the same benefit that a human does from 8 hours of sleep.

The Flameskull's Undead Nature trait, doesn't have this qualifier. As a result, the flameskull does not need to sleep during a long rest, but they do need to spend 8 hours resting, during which they can do no more than 2 hours of "light activity".

So how does the Rejuvenation trait interact with an already begin rest?

So, the question becomes, is their Eldritch Rejuvenation description, linked to their Rejuvenation trait, light activity or not?

The wording of the Eldritch Rejuvenation description is:

A flameskull's shattered fragments reform unless they are splashed with holy water or subjected to a dispel magic or remove curse spell. [...]

The wording of the Rejuvenation trait is:

If the flameskull is destroyed, it regains all hit points in 1 hour unless holy water is sprinkled on it's remains or a dispel magic or remove curse spell is cast on them.

While the flameskull is shattered, it is not a creature (as it is described as being "destroyed"), but instead a collection of shattered bone fragments. Only creatures can rest, therefore, the flameskull must start the rest again once it has reformed.