A party attacks a creature with a lot of hit points and someone casts Sleep. The creature resists the spell because it has more hit points than what was rolled by the caster for the sleep spell, but can the sleep effect be triggered later once the creature loses enough hit points to fall below that sleep threshold?
For example: The party fights an enemy creature with 30 hit points. The wizard casts Sleep, the enemy is the only creature in the area of effect, and rolls 5d8 for a total of 20. The creature resists because it has more than 20 hit points. After a couple attacks, the creature is brought down to 18 hit points and we are still within the 1 minute duration of the Sleep spell. Would the sleep effect then get activated since the creature now has less than 20 hit points?
As context for the question, I saw this scenario play out in a season 1 episode of the Critical Role webcast where a Fomorian first resisted a Sleep spell, only to fall asleep after suffering enough damage, even though the Sleep spell was cast a few turns earlier. I know that the DM Matt Mercer plays a modified version of the 5E rules, especially since season 1 was converted from Pathfinder, but since I've only recently started playing 5E, I actually thought that's how the spell worked until a Wizard in my group cast it last night and we spent time re-reading the spell details.
After further research and discussion, it turns out a second sleep arrow was shot by Vex. So while it did not happen this way in Critical Role, the question still stands on its own since I genuinely thought at the time that it was how the Sleep spell worked in 5E.