Sleep

Sleep is a saving throw now, and it takes two failures to knock someone out

At the table. Sleep hits every creature you choose in a 5-foot-radius Sphere within 60 feet. Each makes a Wisdom save. On a fail it has the Incapacitated condition until the end of its next turn, then saves again; fail that too, and it's Unconscious for up to a minute of your Concentration. Damage, or someone within 5 feet taking an action to shake it, wakes it. Creatures that don't sleep, such as elves, and creatures immune to Exhaustion always succeed.

What changed from 2014. Everything. In SRD 5.1 Sleep had no save: you rolled 5d8 and put that many Hit Points' worth of creatures to sleep in a 20-foot radius, weakest first. It didn't need Concentration. It was brilliant at level 1 and useless against anything with 30 Hit Points. The 2024 version works on big creatures too, but only through two failed saves.

How often it works. Chance of one failed save (Incapacitated) and two (asleep):

The old 5d8 tops out at 40, and the SRD 5.2.1 Ogre has 68 Hit Points, so the old spell could never touch it. Now it's the best target on the list.

The choice that matters is who you put in the Sphere. Even the first failed save is useful: an Incapacitated creature loses its Concentration, so an enemy caster who fails once drops their spell.

The common mistake is hitting a sleeper. Any damage wakes it. Walk up and tie it up, or leave it for later.

The human angle. When a monster casts Sleep on the party, that round of Incapacitated is a round of dread. Berns and colleagues found people will take a bigger shock sooner rather than wait for a smaller one, because the waiting itself hurts. As the DM, keep that round short: say the eyelids are heavy, then move straight to their save.

Source: Berns, G. S., Chappelow, J., Cekic, M., Zink, C. F., Pagnoni, G., & Martin-Skurski, M. E. (2006). Neurobiological substrates of dread. Science, 312(5774), 754–758. doi:10.1126/science.1123721