The sleep spell (PHB 276) states:
This spell sends creatures into a magical slumber. Roll 5d8; the total is how many hit points of creatures this spell can affect.
This and other similar spells like color spray affect a pool of hit points directly, not dealing damage and encountering potential resistances nor requiring a saving throw that may be given advantage via mechanics.
There are creatures that explicitly give advantage on saving throws against being put to sleep magically, though, such as the Bugbear Chief (MM 33), which has the trait Heart of Hruggek:
The bugbear has advantage on saving throws against being charmed, frightened, paralyzed, poisoned, stunned, or put to sleep.
or the Balor (MM 55), which has the trait Magic Resistance:
The balor has advantage on saving throws against spells and other magical effects.
Sleep in this case comes from a magical source, so I believe it falls under the category of a magical effect.
Given these specific wordings, does the general behavior of the sleep spell change to fit the circumstance, giving the targeted creature a saving throw against the caster's DC? If so, what type of saving throw? Or does the spell simply ignore these traits and proceed to affect the hit point pool directly?