D&D 5E Effect of Sleep in swarms of monsters.

[Tormyr]

As an aside on swarms: Last game, I actually got to utter the sentence, "Well, mechanically, this counts as 44 swarms of poisonous snakes."

Oddly enough, they chose not to try and clear the Indiana Jones-esque snake pit.

As one of the PC's in my games would do: Turn tail while saying, "Nope. Nope. Nope!"

 

[Plaguescarred]

I have swarms be affected by conditions in the same way other creatures are, including unconscious. So the sleep spell would work the same way against a swarm than a non-swarm one.