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Temporary hit points in Dungeons & Dragons are a mechanic that allows "a buffer against damage" (PHB 198). There are a few spells of note that can affect a character based on how many hit points they currently have, most notably sleep, power word stun, and power word kill.
Does temporary HP count towards the HP total for determining resistance for these effects?
The rules distinguish temporary hit points as separate and distinct from actual hit points (emphasis mine):
Some spells and special abilities confer temporary hit points to a creature. Temporary hit points aren't actual hit points; they are a buffer against damage, a pool of hit points that protect you from injury.
[...]
Because temporary hit points are separate from your actual hit points, they can exceed your hit point maximum. A character can, therefore, be at full hit points and receive temporary hit points.
As you can see, the rules clearly specify that temporary hit points aren't actual hit points. Thus, spells that affect characters based on their HP, such as sleep ("Creatures [...] are affected in ascending order of their current hit points"), power word stun ("If the target has 150 hit points or fewer, it is stunned."), and power word kill ("If the creature you choose has 100 hit points or fewer, it dies.") don't consider temp HP at all in determining whether the creature is affect.
Rules designer Jeremy Crawford unofficially confirms this answer in a 2017 tweet:
I did my best to search, but couldn’t find anything. Do Temporary Hit Points count for purposes of determining whether or not a creature is affected by Sleep or Power Word Kill?
Hit points and temporary hit points are never added together.
So no, temporary HP is not considered for the effects of those spells unless explicitly stated otherwise in the spell description. It's generally just a damage buffer, nothing more.
I believe you will find your answer here. This is a similar question, but the top answer sounds like it's delving into the very topic your touching on here.
But, I will also attempt to help. There is a difference between both "hit points" and "temporary hit points." It even states it in the PHB PG 198.
Because temporary hit points are separate from your actual hit points, they can exceed your hit point maximum.
There is also the little tidbit you mentioned that Temporary Hit Points are a buffer against damage. The Sleep spell, Power Word Stun and Kill all call for the total number of "hit points" which, as indicated in the above block quote, are separate from Temporary Hit Points. They are also not inflicting damage, and because Temporary Hit Points are only taken into account when taking/dealing damage are not utilized.
