There is nothing quite like the awesome display oracle revelation for sleep and/or deep slumber that I am aware of. There are, however, things that work like sleep or deep slumber but with higher or no HD caps.
Mythic sleep has an 8-HD limit, and mythic deep slumber has a 20-HD limit.
The sandman bard archetype gets a spell-like ability that is “as deep slumber, but with no HD limit,” but that is only when using that particular ability, rather than a change made to the sleep or deep slumber spells that the sandman might cast.
Similarly, the slumber witch hex is “as per the spell sleep” and “can affect a creature of any HD.” Again, that is only while using slumber and does not apply to any sleep or deep slumber spells.
Elf's Fey Ancestry is the closest I can think, but it is still immunity, not "resistance".
Fey Ancestry. You have advantage on saving throws
against being charmed, and magic can’t put you to sleep.
Aside from that, "Resistance" against sleep is something hard to define, to begin with. It is not a damaging spell, so the common "resistance" = half damage doesn't apply.
It also doesn't require an Attack Roll or a Saving Throw, so disadvantage/advantage (respectively) also doesn't apply.
Because of these reasons, I don't think there is any kind of feature that mitigates the sleep spell - every entry I know of is either immune to it or not.
Little Frame Challenge: You want it RAW, but you are yourself using it for a homebrew, so I fail to see the relevance of RAW.
A simple feature that is somehow "equivalent" to Resistance would say something on the lines of
Putting this creature to sleep costs twice from the HP pool.
Spells that induce sleep
The following spells put one or more affected creatures to sleep.
The 0-level bard spell lullaby [ench] (PH 249) receives honorable mention for giving targets that fail their Will saving throws against its effect a −2 penalty on saving throws against sleep effects.
Also, the effect of the 3rd-level Sor/Wiz spell feign death [necro] (Tome and Blood 89) and the effect of the 6th-level bard spell rapture of the deep [ench] (Stormwrack 120) can be easily mistaken for sleep effects even though they aren't, technically, sleep effects.
Note: I suspect the question's goal is an attempt to determine the value of the Dragon #325 feat Potent Enchantment (77). (And, if not, maybe somebody searching for this is.) Player's Handbook spells affected by that feat’s benefit include animal trance, calm animals, hypnotism, and mass charm monster in addition to the aforementioned sleep and deep slumber. Other spells on this list are unaffected by the benefit of the feat Potent Enchantment, either because the affected creature's Hit Dice are fixed like in the symbol and arrow spells or because a creature's Hit Dice don't matter to the spell.