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
- 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.