Well, I can’t simplify more than the title.
This question came from KorvinStarmast’s answer and is related to Viishnahn’s question. The sleep spell states
This spell sends creatures into a magical slumber. Roll 5d8; the total is how many hit points of creatures this spell can affect. Creatures within 20 feet of a point you choose within range are affected in ascending order of their current hit points (ignoring unconscious creatures).
Starting with the creature that has the lowest current hit points, each creature affected by this spell falls unconscious until the spell ends, the sleeper takes damage, or someone uses an action to shake or slap the sleeper awake. Subtract each creature’s hit points from the total before moving on to the creature with the next lowest hit points. A creature’s hit points must be equal to or less than the remaining total for that creature to be affected.
And the Bugbear Chief (from the Monster Manual) has a feature that states:
Heart of Hruggek. The bugbear has advantage on saving throws against being charmed, frightened, paralyzed, poisoned, stunned, or put to sleep.
The thing is: the only sleep inducing effect I am aware of is the sleep spell, which does not require any saving throw. So, this last part of the Bugbear Chief seems useless, since the only effect inducing sleep won’t be affected by his advantage at all.
Sure, it could be there preemptively for future content or even in case someone homebrews such effects, but, other than that, are there any other (official/published) sleep-inducing effects other than the sleep spell? In particular, any that does require a saving throw? These could be poisons, monster or class features, magic items, or other spells that I’m not aware of.
Unconscious
A filtered search of spells and items that include the unconscious condition on Dndbeyond.com yields only two results.
Sleep effects give you the unconscious condition and include language that involves sleepiness. There are at least two spells besides Sleep that do this:
Eyebite has the Asleep effect that can be imposed on a failed Wisdom save:
Asleep. The target falls unconscious. It wakes up if it takes any damage or if another creature uses its action to shake the sleeper awake.
And symbol has the Sleep effect:
Sleep. Each target must make a Wisdom saving throw and falls unconscious for 10 minutes on a failed save. A creature awakens if it takes damage or if someone uses an action to shake or slap it awake.
Both of these spells require saving throws.
For Monsters that can put a creature to sleep, please see PixelMaster’s answer