Are there consistent rules or lore guidelines about what creatures cannot be magically put to sleep?

Question

I've recently become unreasonably annoyed by sleep effects and how the game seems to handle them. There is a story tied to this:

I play a level 11 bard in our bi-weekly 5e game. Our party was recently confronted by one of our reoccurring villains, an unfriendly lich. After an epic battle both the party and the lich were on the brink, with our resources depleted, our wizard possibly dead and the our fighters on the death's door. Luckily our sorcerer was able to rid the monstrous enemy of all his legendary resistances. My bard looks around the corner of the door to see the undead figure, squints the inky voids that he has for eyes thanks to the eyebite spell, and the lich... falls asleep snoring?

Asleep. The target falls unconscious. It wakes up if it takes any damage or if another creature uses its action to shake the sleeper awake.

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It feels like there are creatures and/or creature types in D&D 5e that are designed so that they are immune to sleep. However, the rules don't reflect this as far as I know.

Shadar-Kai monster stat blocks, as well as any PC that plays an elf (including eladrin) have the following feature:

Fey Ancestry. The [XYZ] has advantage on saving throws against being charmed, and magic can't put it to sleep.

yet the actual Eladrin stat blocks, or most fey, don't get this benefit.

Similar examples can be shown for the Warforged race vs. warforged creatures like the Warforged Colossus, or the Reborn lineage vs. our unfriendly Lich.

Are the rules just inconsistent? Is this an oversight? Are there some other guidelines (for example, in the lore of these creatures) that are more consistent in how to approach this?

I'd prefer an answer that uses official D&D 5e materials since that is the version I care about, but if the best answer requires further context of Dungeons and Dragons lore or other editions, I'd be happy with it as well.

Answer 1

There is explicit guidance that there are no such general rules

This is on page 6 of the Monster Manual, under the heading "Type":

A monster's type speaks to its fundamental nature. Certain spells, magic items, class features, and other effects in the game interact in special ways with creatures of a particular type. For example, an arrow of slaying (dragon) deals extra damage not only to dragons but also other creatures of the dragon type, such as dragon turtles and wyverns.

And the "Tags" subsection:

A monster might have one or more tags appended to its type, in parentheses. For example, an orc has the humanoid (orc) type. The parenthetical tags provide additional categorization for certain creatures. The tags have no rules of their own, but something in the game, such as a magic item, might refer to them. For instance, a spear that is especially effective at fighting demons would work against any monster that has the demon tag.

From this, it is clear that the design philosophy in 5e is to not provide general rules based on overarching creature types. They just provide a label for other rules to inteact with. Any specific rules such as not needing to sleep, immunity to the unconscious condition etc. are then listed for the individual race or creature.

For an example of other rules interacting with creature type, the sleep spell says:

Undead [...] aren’t affected by this spell.

If all Undead would be immune to sleep effects, this clause would be superfluous. But since the game has an Undead type, it can use that to make them immune to this sleep effect. The added rules text cost is that now every sleep effect needs to include such text. If you leave it out, it will put Undead to sleep unless they are not immune to sleep based on their individual description. The benefit is that you do not need to learn, look up or remember general rules about Undead to know how the spell works; it's right there.

You can try to infer some patterns; for example, many constructs have a lore entry Constructed Nature. The one in the Animated Objects section says: "An animated object doesn't require air, food, drink, or sleep" and the one in the Golems section says "A golem doesn't require air, food, drink, or sleep", so from this you might build an idea that it is common for constructs to not require air, food, drink or sleep.

But if a construct does not have such a description, these aspects will not apply to that construct. For example, the Homunculus also is a construct, but it does not share that trait and thus needs to breathe, eat, drink and sleep.

So yes, the rules are just inconsistent like that. This is different from earlier editions like 3.5e, where the monster types all had mechanical attributes. There, all constructs did not eat, sleep, or breathe.

Answer 2

There isn't much consensus based on filtering published monster statblocks.

Generally, check for immunity to the unconscious condition (or Fey Ancestry), but there might be complicating factors based on how you're making the target unconscious. The text present on the method will tell you more.

According to the known magical effects specifically described to put a target to sleep, making someone sleep, mechanically, simply makes them unconscious:

Sleep spell:

[...] Starting with the creature that has the lowest current hit points, each creature affected by this spell falls unconscious until the spell ends, the sleeper takes damage, or someone uses an action to shake or slap the sleeper awake.^[1]

Sleep effect of eyebite:

[...] Asleep. The target falls unconscious. It wakes up if it takes any damage or if another creature uses its action to shake the sleeper awake. ^[2]

Sleep effect of symbol:

[...] Sleep. Each target must make a Wisdom saving throw and falls unconscious for 10 minutes on a failed save. A creature awakens if it takes damage or if someone uses an action to shake or slap it awake. ^[3]

So, if they cannot become unconscious, they cannot be made to sleep, period. There aren't many of those; the tool I used showed 36 out of 2,950 listed monster statblocks contained that immunity. They're mostly constructs, elementals, and undead. (This, of course, applies to the unconscious condition generally, including things like knocking a creature out via nonlethal blows, whereas the Fey Ancestry feature only applies to rendering a creature asleep/unconscious via magic.)

This is what the effect of eyebite falls under, as the spell doesn't list any separate factors that would cause the spell to fail. Can it be unconscious at all? Is it missing the inherent protection of Fey Ancestry from becoming unconscious via magic? Cool, it works by RAW.

The sleep spell, of course, tells you the alternate conditions that make the spell fail to work^[1]:

Undead and creatures immune to being charmed aren't affected by this spell.

There are a lot more of those, including the vast majority of constructs, a lot of oozes and aberrations, and a number of high-level fiends and celestials.

(Features such as Constructed Nature don't really intersect with immunity to being unconscious on the mechanical level, leaving the option open for the higher-level spell effects noted here. Since I sleep my computer all the time, I would personally rule that it works so long as the spell doesn't have additional failure conditions the target falls into. I would still recommend discussing with your DM how they interpret this rules interaction.)

Other ways of making something unconscious usually have other mechanisms that introduce other rules--e.g., some poisons induce unconsciousness via the poisoned condition, so any target immune to the poisoned condition would likewise be immune to becoming unconscious in this way.

If all else fails, check the way you are making someone unconscious/putting them to sleep for complicating factors, and if none are present (as in eyebite) weigh the chances of whether the target is likely to be immune to being unconscious at all.

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^{[1] Player's Handbook, p. 276}

^{[2] Player's Handbook, p. 238}

^{[3] Player's Handbook, p. 280}