Elves cannot be magically put to sleep, according to their Fey Ancestry trait.
The catnap spell (Xanathar's Guide to Everything, p. 151) can make up to three willing creatures fall unconscious (and get the benefit of a short rest after the 10-minute duration).
Can elves be affected by the catnap spell?
Catnap is a bit deceptively named because, despite the name and the fact that one of its effects mimics a short rest, Catnap does not actually technically put creatures to sleep. And this is not the only example of a spell/ability confusingly named.1
The important thing to realize about this is that the names of spells/abilities have no mechanical meaning. Only the spell effect description describes how the spell works.
You make a calming gesture, and up to three willing creatures [...] fall unconscious for the spell’s duration.
Nowhere in the spell does it say anything about putting creatures to sleep, it only imposes the unconscious condition on them.2
And, since elves are not immune to the unconscious condition, they are affected by the spell like anybody else is.
Jeremy Crawford has confirmed this interpretation is as intended:
Q: My group is discussing the Catnap spell; are elves unable to be affected by it, even willingly?
A: Elves aren't immune to the unconscious condition.
Q: But they can't be put to sleep by magic. Is the effect of the catnap spell not magic or not sleep?
A: Nowhere does the catnap spell say it puts you to sleep.
1 - A few examples: Sacred flame does not do fire damage. Chill touch does not do cold damage and is also not a touch spell. Daylight does not actually create sunlight. Detect good and evil does not actually detect alignment.
2 - Compare this to sleep which says:
This spell sends creatures into a magical slumber.
Catnap doesn't actually say 'sleep' or 'slumber' or any similar word in the spell effect, so there's no reason an elf would be immune to the unconsciousness it causes.
Your DM might decide to disagree, based on the name of the spell, but this is probably one of those cases where the effects at the table are more important than the strictest reading of the rules. It's good for the party to be able to all reset their abilities at the same time, and bad to exclude one player from a beneficial effect just because of their species choice.
Or to put it another way, there's no harm in letting the elf take a short rest quickly, and there is harm in making the player feel like they're going to miss out, so it's better to say yes.
Sleep is a 1st level enchantment spell (magic) that causes an unconscious condition (sleep)
Sleep does state:
“... the sleeper takes damage, or someone uses an action to shake or slap the sleeper awake.”
Catnap is a 3rd level enchantment spell (magic) that causes an unconscious condition (“unconscious”)
Catnap does state:
The spell ends on a target early if it takes damage or someone uses an action to shake or slap it awake.
awake by English definition is the opposite of sleep
Important note:
Conditions:
- Blinded - Charmed - Deafened - Frightened - Grappled
- Incapacitated - Invisible - Paralyzed - Petrified
- Poisoned - Prone - Restrained - Stunned - Unconscious
It comes down to lack of cohesion with core vs explanation terminology.
The term sleep is one of the ways to acquire the unconscious condition, which is the rules mechanic. Same as being knocked out, instead of killed, when you hit 0 hit points.
So because Catnap doesn’t use the term sleep or slumber in the description, that somehow it’s different. Even though the title refers to a sleep terminology.
Fey Ancestry - ... magic can’t put you to sleep.
Trance - Elves don’t need to sleep
Elves can sleep, they just physically don’t have to.
I would rule, that the spell specifies willing and elves can sleep, that it can effect them. When it comes to magically forcing an elf to sleep, is what this racial trait is intended.
Not really well thought positive benefit as a modified sleep spell, but it is a weak technicality.
